The effect of addition of free non-absorbing polymer [poly(ethylene oxide)(PEO) with M_W= 20 000, 35 000 and 90 000] on the stability of concentrated (40% w/w) sterically stabilised polystyrene latex dispersions has been investigated using rheological measurements. Steady-shear, creep-compliance and elastic-modulus measurements were carried out to obtain the following rheological parameters: plastic viscosity η_PL, extrapolated yield value τ_β, residual viscosity η₀ and shear modulus G₀. With the exception of η_PL all the other parameters showed and increase above a certain critical PEO concentration, which decreased with increase of M_W of the added polymer. This critical concentration was lower than the flocculation concentration obtained using a dilute dispersion. However, at higher PEO concentrations there was some indication of a reduction in η₀ values, indicating reduction in the extent of flocculation. Experiments whereby the PEO concentration was reduced by adding more latex indicated some irreversibility of the flocculation process. Three theories were used to calculate the depth of the minimum in the interaction-free-energy–particle-separation curves in the presence of the free polymer. The value for the free-energy minimum has been used to calculate the yield value and comparison was made with the experimental results.