PTxy data (bubble-point and dew-point pressures at a specific composition) for two binary systems, propane (1) + n-butane (2) and propane (1) + isobutane (3), are presented in this paper. These binary systems are under consideration for use as a refrigerant for a Lorentz cycle. A recirculation method along with the aid of a new composition measurement system was employed. The experimental uncertainty is 3 mK for temperature, 0.027% + 1.04 kPa for pressure, and 0.11 mol % for composition. A total of 60 data points were obtained in the temperature range of (270 to 310) K for propane (1) + n-butane (2) and (260 to 320) K for propane (1) + isobutane (3). Comparisons of the present data with available mixture thermodynamic models and other experimental data are discussed in this paper.