The process of physical aging is accompanied by a gradual change in a number of physical parameters of the solid under study. In this paper we show that by correlating the excess enthalpy with the excess volume, bulk modulus, K, data of reasonable magnitude are obtained for polystyrene, PS, and poly(vinyl acetate), PVAc. The underlying reasoning is based on standard equilibrium thermodynamics. For PVAc the time dependence of K during the aging process is discussed in terms of corresponding data calculated from dynamic mechanical measurements at varying frequency. Examples are also given of calculations of K from uniaxial stress relaxation data obtained with polyethylene samples where both the volume and the concomitant stress changes were determined. The calculation of K was based on the usual formula relating pressure (1/3rd of the uniaxial stress) to the corresponding volume change. The K data obtained were somewhat lower than those normally reported, an effect likely to be related to the time scale involved.