The weak polyelectrolyte sodium polyacrylate, NaPA, and the neutral poly(vinyl pyrrolidone), PVP, were combined and the effects of complexation both in solution and at the nanoparticle interface were studied under basic pH conditions. Using a combination of SANS and PCS, we demonstrate that attractive inter-polymer interactions occur between PVP and NaPA in solution (for polymers with molecular weights equal to 40 K and 60 K respectively); typically, no attractive interactions between PVP and NaPA are reported above a critical pH of 4–5. Polymer interactions in the bulk are observed between the larger molecular weight polymers studied. The mass ratios of polymer also affect the interactions in the bulk, indicating that polymer interactions are influenced by the charges present in the system. The addition of NaPA to a silica dispersion with PVP previously adsorbed to the particles is shown to led to polymer desorption and the destabilisation of the system leading to particle aggregation or even flocculation. SANS data show that the interparticle potential changes from being repulsive with bare silica or PVP-coated silica to attractive on addition of NaPA. The molecular weight of PVP is seen to affect the amount of polymer desorbed, whereas the molecular weight of NaPA had an effect on the aggregation of particles.