An experimental method was presented for vapor liquid equilibrium measurement by improving the PVT measurement apparatus. Isothermal vapor liquid equilibrium data for pentafluoroethane (HFC-125) + isobutane (HC-600a) were measured at (243.15, 253.15, 263.15, 272.15, 283.15, 293.15, 303.15, 313.15, 323.15, and 333.15) K at pressures up to 3.2 MPa. The experimental data were correlated with the Peng-Robinson equation of state with van der Waals mixing rules. The overall average absolute deviation between measured pressures and calculated results from the Peng-Robinson (PR) equation of state is 0.87 %. The interaction parameters k(12) were obtained for different temperatures, and the values of k(12) increase from 0.12 to 0.22 as the temperature increases.