A new technique is described that has been used for the first time to measure microcrack densities, size distributions, and aspect ratios in rocks at elevated effective pressures. The distribution of microcracks in Coconino sandstone has been examined quantitatively at effective pressures to 34.5 MPa. The mean crack thickness and aspect ratio decreased monotonically with increasing effective pressure. This was accompanied by a significant reduction in crack porosity. However, overall crack density changed most between 0 and 3.45 MPa and remained nearly constant at high pressures. The abundance of the relatively thick microcracks (0.5--4 μm) decreased most with increasing pressure, suggesting that changes in seismic quality factor Q and velocity with increasing pressure may be related primarily to the closure of the relatively thick cracks with high aspect ratio, as opposed to the relatively thin cracks with low aspect ratio. ¿ American Geophysical Union 1987