The critical heat flux (CHF) in forced convective upflow has been investigated at different compositions of the binary mixture chosen, namely R12 (CCl2F2) and R114 (C2Cl2F4). The test section used consists of a vertical stainless steel uniformly heated tube, 2.1 m long, 7.57 mm ID. The experimental results show an almost linear dependence of the CHF on the composition of the mixture even though deviations from linearity have been detected as a function of thermal hydraulic conditions. A further evaluation of the boiling crisis mechanisms and their relation with the CHF has been performed. A link between boiling crisis mechanisms and the functional dependence of the CHF on the mixture composition has been found. Finally, a comparison of experimental data with available correlations is accomplished.