Densities of thermally annealed copolymers were measured at 75° and 100°C at pressures to 1 kilobar. The compositions ranged from pure polybutadiene to pure polystyrene, from completely random to completely block copolymers. The experimental results are well represented by the Tait equation. To a first approximation, specific volumes and compressibilities are linear functions of weight per cent styrene. For a fixed composition, the effect of structure on volumetric properties is small. However, it appears that, when compared at the same overall composition, the specific volume and compressibility are larger for the random copolymer; specific volume and compressibility decline slightly when the styrene is in block rather than random form.