In the precipitation circuit of the Bayer process, the organic impurity sodium oxalate (Na2C2O4 ) co-precipitates with the product - gibbsite (γ-Al(OH)3). This co-precipitation, results in numerous industrial problems such as increasing the content of gibbsite fines, decreasing gibbsite agglomeration efficiency, increasing the rate of oxalate-gibbsite scale formation and increasing sodium contamination of the alumina product. In this thesis, the investigation of the co-precipitation process has been broken down into four distinct parts: (1) the morphological control of Na2C2O4 crystals, (2) Na2C2O4 precipitation on gibbsite crystals, (3) the growth mechanism of gibbsite crystals and (4) gibbsite precipitation on Na2C2O4 crystals and their intergrowth. The morphology of Na2C2O4 crystals grown under industrial Bayer conditions is variable as many process parameters including sodium hydroxide concentration and inorganic impurity species can have a significant influence. Sodium hydroxide is the most important solute in Bayer solution; this study shows how a high concentration has a dramatic effect on reducing the size of Na2C2O4 crystals and increasing the surface roughness of the (110) faces. The morphology of the Na2C2O4 crystals was also shown to depend on the nature and concentrations of the inorganic anion species present. The presence of monovalent anions (OH-,Cl-,Al(OH)4-) promoted flower-like oxalate crystals constructed by the rod-like units with different widths and lengths. The presence of divalent anions (CO32-,SO42-) leads to many different oxalate structures including cyclic twins, spherulites and double-leaf structures. Due to these morphological modifications, divalent anions in the Bayer liquor can largely increase the specific surface area of Na2C2O4 crystals. Increased surface area provides more sites for secondary nucleation of gibbsite, increasing gibbsite fines. In gibbsite precipitation tanks, the suspended gibbsite seed provides very high surface areas for sodium oxalate nucleation and growth, which is the first step for co-precipitation of gibbsite and sodium oxalate. Sodium oxalate tends to nucleate on gibbsite crystals and grow with a needle-shaped morphology which eventually develops into oxalate bundles. Caustic washing was able to partly decompose gibbsite crystals and thus make the gibbsite surface rougher, which provides more sites for oxalate nucleation and growth. In situ atomic force microscopy (AFM) was used to study the nucleation behaviour of sodium oxalate on gibbsite substrates. The growth of oxalate three-dimensional (3D) islands was observed on gibbsite (001) faces at medium oxalate supersaturation (σ=(Cinitial-Cequlibrium)/Cequlibrium=1.2). There is a tendency for the 3D islands to be preferentially oriented with their (200) twin planes parallel to the gibbsite substrate (001) faces confirmed by both the calculation of lattice mismatch and AFM images. The formation of heteroepitaxial layers of sodium oxalate on the gibbsite (100) faces were observed at high oxalate supersaturation (σ=1.4). The total strain energy (γtotal) in this oxalate layer was 6.62 mJ/m2 smaller than the interfacial tension of the gibbsite-solution interface (γgs) indicating that oxalate growth on gibbsite (100) faces is governed by Frank-van der Merwe growth mode. In the same experiments, oxalate 3D islands form on the heteroepitaxial layers, suggesting the Stranski-Krastanow mode for oxalate epitaxial growth. In order to understand the features of gibbsite nucleation and growth on the sodium oxalate surface, the gibbsite growth mechanism on a gibbsite substrate needed to established first. Using in situ atomic force microscopy, it was found that the growth of gibbsite crystals is controlled by layer-by-layer mechanism through the advancement of micro/macrosteps on basal (001) face and prismatic (100) face. Two dimensional islands often form on the terrace adjacent to the edges of micro/macrosteps. Once sodium oxalate precipitates in gibbsite precipitation tanks, gibbsite begins to nucleate and grow on the surface of the Na2C2O4 crystals. Most of the gibbsite nuclei can break away from oxalate surface, thus influencing the product particle size distribution. Gibbsite nucleation on sodium oxalate crystals and their intergrowth was studied in three distinct stages using in situ AFM and ex situ SEM. In a solution saturated with oxalate and supersaturated in aluminate, gibbsite secondary nucleation occurred and the nuclei were preferentially located along macrosteps of sodium oxalate (110) faces and appeared as two dimensional (2D) islands with pisolitic shapes. The layer-by-layer growth mechanism could be responsible for the morphology of 3D gibbsite aggregates on sodium oxalate (110) faces as well. There does not appear to be a preferred epitaxial relationship at the hetero-interface between gibbsite crystals and sodium oxalate substrates, indicating that the magnitude of the surface free energy for nucleation is greater than that associated with strain energy caused by lattice mismatch. Following gibbsite secondary nucleation, in a solution saturated in aluminate and supersaturated in oxalate, the inclusion of gibbsite crystallites into the bulk of sodium oxalate crystals through step growth on (110) faces of sodium oxalate was observed.