Chitosan (CHT) interacts with proanthocyanidins (PAC) by a mechanism involving hydrogen bonding and ion-dipole interactions, allowing the spontaneous formation of PAC-CHT composite nanoparticles (PAC-CHT NPs). The interaction between PAC and CHT was characterized by ellipsometry, infrared spectroscopy, and surface plasmon resonance (SPR) to determine the effect of CHT molecular weight (MW), PAC to CHT ratios, and pH on the formulation of PAC-CHT NPs. These parameters also affect the size and morphology of PAC-CHT NPs. Results indicate that CHT MW and pH of the solution impact the interactions of PAC-CHT in two ways: (1) greater CHT MW increases the amount of PAC molecules that attach to the CHT chain, and (2) lower pH of the CHT solutions increases the amount PAC molecules that attach to the CHT chain. Results also show that higher CHT MW, CHT concentration, and pH of the CHT solutions increase the size of PAC-CHT NPs. In contrast, greater PAC concentrations decreases the size of PAC-CHT NPs. This study demonstrates that SPR is a useful technique for measuring the effect of changes in the interaction between PAC and CHT, which in turn affects the size and morphology of PAC-CHT NPs.