In the laboratory, we determined the pore-compressibility characteristics of some limestone samples obtained from a Saudi Arabian petroleum reservoir. The samples were saturated with water and subjected to confining pressure, p(c), and pore pressure, p(p), in a core holder, and the resulting reduction in PV was measured, We used differential pressure, p(c)-p(p), ranging from 0 to 4,500 psi. Differential pressure was increased incrementally by reducing the internal (pore) pressure while maintaining a constant hydrostatic confining pressure on the rock samples. We present the experimental data here in the form of relative porosity and hydrostatic pore-compressibility values as functions of differential pressure. We analyzed the test results after grouping the rock samples on the basis of their lithological description. Four groups of limestone samples consisting of grainstones, packstones, wackestones, and mudstones were studied. Also, Berea sandstone was tested for comparison with limestone. The results show linear negative correlations on a log-log scale between pore compressibility and differential pressure for all the tested samples. Consequently, we computed the pore compressibility vs. differential-pressure power-fit parameters (referred to here as parameters A(c) and B-c). We found that parameters A(c) and B-c are correlated with initial porosity for the tested samples and inter-related within the same lithologic group. Mathematical expressions for predicting pore compressibility and porosity as functions of differential pressure are given for the tested samples.