[en] Experiments have been performed using vertical heated tubes, cooled internally by Freon-12, to determine critical heat fluxes (CHFs) for both a uniformly heated section and an exit region with a separately controlled power supply. Heated lengths of the main separately were 2870 mm (8.48 and 16.76 mm tube bores) and 3700 mm (for 21.34 mm tube bore); heated length of the exit section was 230 mm. Coolant pressures, exit qualities and mass fluxes were in the range 0.9 to 1.3 MPa, 0.19 to 0.86, and 380 to 2800 kg m^-2s^-1, respectively. The data have been compared with published empirical correlations specifically formulated to predict CHFs in Freon-cooled, vertical tubes; relevant published CHF data have also been compared with these correlations. These comparisons show that, even over the ranges of conditions for which the correlations were developed, predicted values are only accurate to within +-20 per cent. Moreover, as mass fluxes increase above 3500 kg m^-2s^-1, the modified Groeneveld correlation becomes increasingly inadequate, and the Bertoletti and modified Bertoletti correlations under-predict CHF values by increasing amounts. At mass fluxes below 750 kg m^-2s^-1 the Bertoletti correlations exhibit increasing inaccuracy with a decrease in mass flux. For non-uniform heating, the correlations are at variance with the experimental data