Pool boiling data for binary mixtures of R-22 /R-124 on plain tubes are reported at reduced pressures of 0.1, 0.15, and 0.2. Significant reductions of heat transfer coefficients for mixtures are found as compared to pure refrigerant, and the reduction of heat transfer coefficient is especially pronounced in the vicinity of the pure component. This phenomenon is strongly related to the nonlinear variation of physical properties of liquid viscosity and latent heat of the mixtures. The available empirical and semiempirical correlations developed for mixtures are compared with the present data. In addition, a correlation based on the present R-22 / R-124 experimental data is proposed. This correlation not only predicts the deterioration of heat transfer coefficients for the present data with success but also predicts the experimental data from other investigators with reasonably good accuracy.