Time domain reflectometry (TDR) is a widely used technique for measuring soil water content. To examine its applicability to rock, the authors applied TDR to a variety of rocks in the laboratory. The obtained relationships between the apparent dielectric constant and the volumetric water content showed a behavior that can be partly explained by the effect of gaps between the TDR probe and rock. For rocks with an effective porosity smaller than that of soil, the well-known model presented by Topp et al. <1980> for soil tended to overestimate the water content. For such rocks it was determined that an individual calibration was needed for accurate determination of water content. If no experimental calibration is possible, frequently used mixing models may be used to establish the calibration function. Our investigations suggest that TDR showed the potential for measuring the apparent dielectric constant of rock and, further, for determining water content in rock. ¿ 1998 American Geophysical Union