Seismic velocities were measured in the laboratory on 67 samples as a function of pressure and temperature when saturated with gas and a 35 degrees API [ degrees API = 141.5/(density at 1 atmosphere and 60 degrees F) - 131.5] mineral oil and flooded with pentane and fresh water, respectively, using an ultrasonic pulse transmission method. The rock samples consisted of 39 carbonate cores from eight reservoirs, 22 sandstone cores from six formations, and six unconsolidated samples from three origins. The results show that water flooding increases the compressional velocity V p by more than 5 percent in most of the gas-saturated cores at effective pressures (overburden pressure minus pore pressure) less than 20 MPa. Such V p increase is less than 5 percent in about half of the carbonate cores and some of the sandstone cores with normal pentane and in all the consolidated cores (both carbonates and sandstones) saturated with the 35 degrees API oil. However, in unconsolidated samples saturated with gas, pentane, or oil, water flooding increases the V p substantially.Both the laboratory results and theoretical analysis show that seismic methods may succeed in monitoring water floods in some reservoirs under certain conditions, while they may fail in others. It is thus important to carry out petrophysical and feasibility studies prior to the implementation of in-situ seismic monitoring of water-flood processes.