Summary. The presence of clay minerals can alter the elastic behaviour of rocks significantly. Although clay minerals are common in sedimentary formations and

Compressional-wave velocity (V p ) and quality factor (Q p ) have been measured in Berea and Michigan sandstones as a function of confining pressure (P c ) to 55 MPa and pore pressure (P p ) to 35 MPa. V p values are lower in the poorly cemented, finer grained, and microcracked Berea sandstone. Q p values are affected to a lesser extent by the microstructural differences. A directional dependence of Q p is observed in both sandstones and can be related to pore alignment with pressure. V p anisotropy is observed only in Berea sandstone. V p and Q p increase with both increasing differential pressure (P d = P c -P p ) and increasing P p . The effect of P p on Q p is greater at higher P d . The results suggest that the effective stress coefficient, a measure of pore space deformation, for both V p and Q p is less than 1 and decreases with increasing P d .