Abstract In this work, a Helmholtz free energy equation of state for R290 + R134a based on the multi-fluid approximations model was developed to describe the thermophysical properties of the binary mixture. The present equation shows more competitive than the default equation in REFPROP 9.1 (Lemmon et al., 2013). It covers a temperature range of (252–400) K and pressures up to 6.1 MPa. The estimated standard uncertainties of the present equation are 0.70% and 0.90% for the bubble point and dew point pressures, 0.26 K and 0.29 K for the bubble point and dew point temperatures, 0.13% and 0.60% for the saturated liquid and vapor densities, respectively. The main purpose is to provide a useful model for R290 + R134a, and thus to promote its better applications for the design and optimization of the refrigeration systems.