Laboratory studies of the detailed relationships between acoustic properties and the petrographic character of brine- and air-saturated carbonate rocks with a wide range of facies, porosities, lithologies, and rock fabrics indicate that porosity is the major factor influencing both P- and S-wave impedance and velocity. Primary lithology and secondary mineralogy have only a small influence on impedance and velocity. Combined use of P- and S-wave velocity data discriminates porosity changes from lithologic changes. All other variables, including pore-fluid type and petrographic fabric, have no significant influence on velocities.Laboratory measurements of P-wave velocity under simulated in-situ conditions reproduce well-log velocity values reliably. Laboratory porosity-velocity trends agree with the time-average equation when the correct matrix velocities are used. Rock property results were used to interpret porosity/lithology variations for an inverted seismic section from the Williston basin. Where well control was available, the porosity/lithology interpretation was found to be in agreement with the subsurface control.