By analyzing experimental data we show that in many room-dry rocks, the Poisson's ratio (PR) decreases with decreasing differential pressure (confining minus pore pressure). This means that in gas-saturated rocks, PR decreases with increasing pore pressure. We confirm the generality of the observed effect by theoretically reproducing it via effective medium modeling. This effect can be used as a new tool for overpressure detection from surface seismic, cross-well, sonic logs and measurements ahead of the drill bit. ¿ 1999 American Geophysical Union