The effect of porosity on several material constants can be described by three types of empirical functions. One of these features linear increase from zero to unity of the relative bulk material constant when the relative density increases from &pgr;/6 to 1. An almost identical function results from application of a theoretical model with the following properties.(1) The porous compact is divided into two systems: first, isolated pores in a continuous solid matrix; second, solid grains mixed with continuous voids.(2) The geometry of both systems is assumed to be a cubic array of equal spheres. With this simplification, the relative bulk material constants of each system are calculated as functions of the relative density. The equations obtained are valid for those material constants which are ratios of corresponding fluxes and forces in the sense defined by Maxwell (e.g., the electric conductivity).(3) Weight factors for the presence of either system are introduced.(4) The weight factor for the system of isolated pores is assumed equal to the relative bulk material constant of the total compact for all values of relative density.The implications of this model to the mechanism of sintering are discussed.