For 1-pentene and the three branched C5H10 isomers (2-methylbut-1-ene, 3-methylbut-1-ene, and 2-methylbut-2-ene), vapor pressures and vapor liquid critical properties were determined, thus filling several gaps in the thermodynamic database for these compounds. Using a static titanium autoclave with sapphire windows for high temperature and pressure determinations, vapor pressures and critical temperatures, pressures, and density data were measured. The experimental data were used to fit parameters for the Wagner vapor pressure equation and compared to available literature data. The consistency of the critical values was checked with the critical surface equation.