The moduli and Poisson ratio of polystyrene-polyethylene blends with different compositions have been determined. In some cases a polystyrene-polyethylene copolymer was added to the blends to provide for adhesion between the components. It is shown that the Kerner packed grains model can be used to predice accurately both modulus and Poisson ratio of these blends. Deviations of both modulus and Poisson ratio from the predicted values occur in some blends containing block copolymer as a consequence of the copolymer-induced formation of a continuous low modulus phase at a relatively low concentration of the low modulus material. In the case of non-adhesion between the components only the Poisson ratio was found to deviate significantly from the predicted value. This is explained by assuming hole-like behaviour of the polyethylene particles due to non-adhesion and the misfit of the coefficients of thermal expansion of polystyrene and low density polyethylene.