Interactions between poly(styrenesulfonate) (PSS) and poly(acrylic acid) (PA) in aqueous solution have been studied, with and without added salt, and at various degrees of neutralization (alpha) of PA. Equilibrium phase diagrams have been determined, and the viscosities of monophasic mixtures have been measured. Both types of experiments reveal striking effects of alpha on the PA-PSS interactions. Salt-free mixtures with fully or partially neutralized poly(acrylic acid) phase separate segregatively, except at very low alpha where, instead, an association between PA and PSS occurs. The association is evidenced by a dramatically increased viscosity, relative to solutions of PA or PSS alone, in semidilute mixtures. Addition of salt (1 M NaCl) results in an increased two-phase area at all alpha, and in the appearance of an associative phase separation for non-neutralized PA. The qualitative phase behavior observed in the presence of salt can be generated by calculations using the Flory-Huggins theory, if it is assumed that both the PSS-PA and the PA-solvent interactions change monotonically with alpha in a specified fashion. All experimental observations, and the theoretical modeling, suggest that the effective PA-PSS interaction changes (over a narrow interval of alpha) from an attraction at very low alpha to a repulsion at higher alpha.