Abstract Macroaggregates in the northern Adriatic, presently considered as a product of agglomeration of macromolecular dissolved organic matter (DOM) into macrogels and particulate organic matter (POM), are primarily associated with phytoplankton, mostly diatom, blooms occurring in late spring during favorable environmental conditions. Composition of mucous macroaggregates in the northern Adriatic, collected at different evolutionary stages, was studied using spectroscopic (solid-state 13 C NMR, FT-IR and 1 H NMR), elemental and X-ray (XRD) analyses. The results confirm the general structure of macroaggregates, including four major classes of structural elements: carbohydrates, ester and amide functional groups, aliphatic, and organosilicon components. The water-soluble fraction of macroaggregates is mostly composed of carbohydrates with a minor content of aliphatic components, and is similar to ultrafiltered dissolved organic matter (UDOM) from the cultured diatom Cylindroteca fusiformis , suggesting the importance of diatom exudates as macroaggregate precursors. The aliphatic and organosilicon components prevailed in the water-insoluble fraction. The macroaggregate gel is mainly stabilized by organo-mineral interactions with calcite, quartz and clay minerals. The results also indicate the preservation of an N containing component within the macroaggregate refractory organic matrix.