Comprehensive (p, rho, 7) measurements on propane, propylene, n-butane, and isobutane have been carried out in the homogenous gas and liquid phases and along the (vapour + liquid) coexistence curve. The results cover a temperature range from (95 to 340) K at pressures up to 12 MPa. The measurements were performed by using an accurate two-sinker densimeter based on the Archimedes' buoyancy principle. The total uncertainty of the measurements in density is (0.01 to 0.02)% (level of confidence 95%), except for low gas densities. Values for the second and third virial coefficients have also been determined from the measurements in the homogeneous gas regions of the fluids. The experimental results have been compared with previous results of other experimentalists and with values calculated from current equations of state. Moreover, previously unpublished results of (p, rho, T) measurements on propane in the temperature range from (340 to 520) K at pressures up to 30 MPa are listed in the appendix. (C) 2004 Elsevier Ltd. All rights reserved.