The presence of rigid inclusions in a powder compact leads to a reduction in the densification rate of the compact and may also lead to processing defects. In this paper, the densification rate and the constitutive parameters of both homogeneous YBa2Cu3O6+x and composite powder compacts (YBa2Cu3O6+x powder with 10 vol% dense inclusions of YBa2Cu3O6+x) are reported. A small amount of liquid phase, which formed during sintering, was present in the samples. However, even with the presence of a liquid phase, the addition of inclusions still reduces the densification rate of the composite and increases its viscosity. The results have been compared with a published analysis of the problem using measured values of the constitutive parameters. Both the viscosity and viscous Poisson's ratio of the porous body have been measured.