Nanoparticles made of gum arabic and chitosan (degree of acetylation 5) at weight ratio of 1 (scale bar=200nm). Display omitted ► Gum arabic–chitosan nanostructures. ► Chitosan degree of acetylation (DA) determines the structure of the complexes. ► Gum arabic and chitosan with high charge density (DA 5%) form nanoparticles of 200nm. ► Using chitosan with higher DA (25%), soluble complexes are obtained. ► Soluble complexes are flat when adsorbed on mica surfaces (height⩽10nm). The interaction of gum arabic (GA) with chitosan (Ch) of different degree of deacetylation was studied by turbidity measurements, dynamic light scattering and atomic force microscopy. The structure of the complexes was found to be directly related to the charge density of chitosan molecules. Gum arabic and chitosan with a degree of deacetylation of 75% form soluble complexes with a loosely globular structure of about 250nm, at weight ratios up to 1.2, if the concentrations are kept low (total biopolymer concentration up to 0.06%). If chitosan has a higher charge density (degree of deacetylation of 93%), colloidal particles are formed, independently of the polymer concentration or ratio. At low concentrations and GA/Ch ratios of 1 or 1.2, the particles have diameters of 200–250nm. The formation of soluble complexes is attributed to a chitosan lower charge density and the presence of non-charged monomers, which prevent the efficient self-assembly of the macromolecules.