The results obtained point to a relation between the possibility of employing the rheological model (1) for describing the viscous flow of a material and the validity of the Mackenzie equation. Two limiting cases may arise. In one of them, where pore healing in a material occurs through a mechanism of viscous change in shape of the nonporous part of the material, the Mackenzie relation (2) or its generalized variant (15) is applicable. The rheological model for an isotropic solid is described by Eq. (1). When the Nabarro-Herring diffusional creep mechanism is operative, the coefficients of viscosity are calculated with Eqs. (19) and (20). In the other limiting case the nonporous part of a material possesses no viscosity (n = ∞), which rules out the possibility of employing the rheological model (1) and the Mackenzie equation. To describe the kinetics of shrinkage under the action of external pressure and surface tension forces, recourse must be had to Eq. (14). The coefficients of volume viscosity for typical porous structures (one pore and an assembly of pores in a polycrystal grain and pores at grain boundaries) are given by Eqs. (22), (27), and (31).