For the four aliphatic C4H8 isomers (1-butene, isobutene, cis-2-butene, and trans-2-butene), densities, vapor pressures, and critical constants were measured, thus filling several gaps in the thermodynamic data for these compounds. Densities of the four isomers were measured with a computer-controlled high temperature, high-pressure vibrating-tube densimeter system (DMA-HDT) in the subcritical and supercritical states. The densities were measured at temperatures from 273 to 523 K and pressures up to 40 MPa (1725 overall data points), for which densities between 83 and 677 kg/m(3) were covered. The uncertainty in density measurement was estimated to be better than 0.1% in the liquid and compressed supercritical states. Near the critical temperature and pressure, the uncertainty increases to a maximum of 1%. Using a variable volume apparatus with a sapphire tube, vapor pressures and critical data were determined. Vapor pressures were measured from about 280 K up to near the critical point with an uncertainty of +/-(0.1 kPa + 0.001P kPa and 0.07 K). The critical points were determined visually with an uncertainty of 1% for the critical volume, 0.1 K for the critical temperature, and 5 kPa for the critical pressure. The new vapor pressures and compressed liquid densities were correlated with the TRIDEN model. The new data along with other literature measurements were used to develop fundamental Helmholtz equations of state and are reported by Lemmon and Ihmels [E.W Lemmon, E.C. Ihmels, Fluid Phase Equilib. 228-229 (2005) 173-187] in part II of this project. (c) 2004 Elsevier B.V. All rights reserved.