The stability of sterically stabilized polystyrene latex has been investigated in the presence of poly(acrylic acid) (PAA) at various pH values. Flocculation (aggregation) has been monitored by microscopy for a series of samples with various PAA concentrations at fixed latex volume fraction and pH values. It was found that at low pH values (<4) bridging flocculation occurred at a low critical flocculation concentration of PAA, but no restabilization was found, even when the PAA concentration (weight fraction) reached 0.10. At pH >7, three different phase regions could be established on the phase diagram, namely, depletion flocculation at lower PAA concentrations (W(p)+ < W(p) < W(p)++ where W(p) is the weight fraction of PAA), depletion stabilization at higher PAA concentration (W(p)++ < W(p) < W(p)se) and a secondary unstable region (W(p) > W(p)se), where phase separation occurs, at very high PAA concentrations. The molecular weight has little influence on depletion restabilization at higher pH. However, it influenced the critical bridging flocculation concentration (CBFC) and critical depletion flocculation concentration (CDFC), i.e. the larger the PAA molecular weight the lower the critical flocculation concentration of polymer. Rheological data showed the same CFC as the microscopy results at the secondary unstable region. The effects of particle size, latex volume fraction, PAA molecular weight, and PAA concentration on viscoelastic properties of the suspensions at pH = 8.0 and 9.0 were also studied.