An experimental study was carried out on boiling heat transfer of mixtures of R11 and R113 flowing in a uniformly heated vertical tube. In the nucleate boiling region at low quality, mass velocity had no appreciable effect on boiling curves and the mixtures showed lower heat transfer coefficients than either those of an equivalent single component with the same physical properties as the mixtures; or those predicted by a linear interpolation between their single-component values as a function of composition. In the two-phase forced convection region at very high quality, neither reduction of heat transfer coefficients of the mixture nor heat flux dependence was noticeable; unless heat flux was so high as to initiate boiling in a liquid film. An empirical equation was given to correlate the present data both in nucleate boiling and in two-phase forced convection. Critical heat flux (CHF) of the mixtures and the quality at CHF-point were predicted using the mixture physical properties from the Katto correlations established as being applicable to single-component liquids.