Since the introduction of medical computer tomography (CT) in geological sciences, many applications have been worked out. However, its limited resolution (optimal 60¿60¿1000 &mgr;m3) restricts its general use. Enhanced resolution is achieved with synchrotron X-ray tomography, but these instruments are expensive and have a limited access. Another technique is microfocus X-ray tomography of which the advantages will be discussed. This instrument is more accessible than synchrotron facilities. It furthermore has a superior resolution (present-day 10¿10¿10 &mgr;m3) compared to medical CT. In this study, a thorough calibration is performed on samples of 8 mm in diameter and fracture widths from 0 to 400 &mgr;m. A statistical evaluation shows that the beam hardening artifact can be neglected and the standard deviation on aperture measurements of 100 &mgr;m is about 15 &mgr;m. The application of quantitative fracture aperture analysis will be demonstrated on a coal sample whereby a good correlation (r=0.94) with fracture analysis by reflected light microscopy is illustrated. ¿ 2001 American Geophysical Union