Tight gas sands account for approximately 19% of the total U.S. gas production, and may contain more than 35% of the U.S. recoverable gas resources (Oil and Gas Investor, 2005). In order to test technology for the production of gas from tight gas sands, the U.S. Department of Energy sponsored a Multiwell Experiment (MWX) in the Rulison gas field in the Piceance Basin, Colorado (Northrop et al., 1983). Drilling commenced in 1981, the field program was completed in 1988, and reporting was completed in 1990. In this article, we examine the relation between acoustic-wave velocities and pore pressures using data acquired during this experiment, and show how acoustic velocities can be used to estimate pore pressure. This is important, because pore pressure is an important factor determining the amount of gas in place, but is difficult to measure directly in low-permeability formations.