The formation of electrostatic complexes involving a napin protein isolate (NPI) and carboxylated alginate (AL) and sulfated (κ-, ι-, λ-type carrageenan) (CG) polysaccharides was investigated at a biopolymer mixing ratio of 10:1 (NPI/polysaccharide) as a function of pH (4.0–12.0) using turbidity and electrophoretic mobility. The functionality of the ensuing complexes was tested on the basis of their solubility, emulsion stability and foaming capacity and stability relative to NPI alone. Complexation follows two pH-dependent structure forming events associated with the formation of ‘soluble’ and ‘insoluble’ complexes. Soluble and insoluble complexes for NPI–AL, NPI–κ-CG, NPI–ι-CG and NPI–λ-CG mixtures occurred at pHs 7.1 and 6.2, 8.6 and 7.0, 9.5 and 9.3, and 9.0 and 8.7, respectively. Complexation resulted in a shift in net neutrality from 5.0 for NPI alone to pH 4.2, 3.7, 3.2 and 2.3 in the presence of κ-CG, ι-CG, λ-CG and AL, respectively. Solubility and foaming capacity of NPI were reduced with the addition of polysaccharide. Foaming stability was similar for NPI–κ-CG and NPI–λ-CG mixtures relative to NPI, but increased and decreased for NPI–ι-CG and NPI–AL mixtures, respectively. Emulsion stability was found to be similar for all mixtures relative to NPI, except for the NPI–ι-CG mixture which had reduced emulsion stability.