The volumetric response of fluid-saturated and linearly elastic rock due to a change of either mean stress or pore pressure is characterized by three independent material parameters. The unjacketed bulk modulus is a convenient choice because it can be directly measured in a laboratory test under a loading that preserves the difference between the mean stress and the pore pressure constant. For a monomineralic rock, the measurement of the unjacketed bulk modulus is ignored because it is assumed to be equal to the bulk modulus of the solid phase. To examine this assumption, we tested porous sandstones and limestones mainly composed of quartz and calcite, respectively, under the unjacketed condition. Special attention was dedicated to reaching full saturation ensuring the transmission of the pore pressure to the solid frame. The presence of microscale inhomogeneities, in the form of nonconnected (occluded) pores, was shown to cause a considerable difference between the unjacketed bulk modulus and the bulk modulus of the solid phase. Furthermore, we found the unjacketed bulk modulus to be independent of the unjacketed pressure and Terzaghi effective pressure and therefore a constant.