Modified classical compressibility techniques are described which measure both the adiabatic and isothermal bulk moduli of polymers. These differ from familiar methods principally in the utilization of sensitive electronic deformation sensors (differential transformers) to assess volume changes, the autographic recording of pressure and volume changes during an essentially quasi-static pressurization cycle. The uniaxial and volumetric methods both characterize the compressibility behavior of polymers in the relatively low pressure region (0–100 atm.). Compressibility data for several well-known liquids and solids are reported. The nonlinear nature of the PV behavior of liquids are shown to be adequately characterized by a simple equation of state which is applicable to rubbery polymerics as well. Equations of state for polymers and filled polymers containing dispersed void inhomogeneities are discussed. Relationships based upon the infinitesimal theory of the deformations of an externally pressurized hollow sphere are compared with more complex equations of state which consider finite deformations at the center of a hollow sphere. It is concluded that the experimental data for the compressibility of voided solids containing less than 0.5% of initial void volume are best represented by the simpler equations of state derived from infinitesimal theory.