Data examined in this study are previously published laboratory shear (S) and compressional (P) wave velocity measurements on water-saturated sandstone, calcareous sandstone, dolomite, and limestone cores, as well as laboratory porosity measurements on the sandstone and limestone cores. Sandstone and limestone porosities range from .092 to .299 and from .006 to .229, respectively. Differential pressure was varied from 500 to 6000 psi, corresponding to approximate burial depths from 290 to 3460 m, respectively.Sandstone, limestone, and dolomite are effectively separated by Poisson's ratio sigma or, equivalently, by the ratio of P- to S-wave velocity. Separation of sandstone and limestone appears to result from the difference in sigma of the matrix material, namely, quartz (.056) and calcite (.316), respectively.An empirical function, 1/V = A + Bphi , was fit by regression analysis to sandstone and limestone velocity (V s and V p ) versus porosity (phi ) values at each differential pressure. In this equation A and B are constants at each pressure, A being approximately equal to the reciprocal matrix velocity. Decreasing standard deviation indicates that the equation becomes an appreciably more accurate representation of the measured data as pressure increases. Average values of A are near reciprocal velocities of quartz (sandstone averages) and calcite (limestone averages). The constant B, rate of change of reciprocal velocity with porosity, is a critical measure of the sensitivity of velocity to porosity, hence the usefulness of velocity in estimation of porosity. Sandstone S-wave B values are from 2 to 5 times greater than all other values, indicating that sandstone S-wave velocity is by far the most sensitive to porosity variation. Least sensitive is limestone P-wave velocity.