Abstract An experimental study has been carried out for the critical heat flux (CHF) during the natural convective boiling of R-113 at various pressures in a vertical channel. The pressure ranges from 1 to 3 bar and the channel gap-size from 0.3 mm to 2.5 mm as its height is kept constant. Data for pool boiling are also obtained. At all pressures investigated, the CHF decreases as the gap-size is reduced. On the other hand, for this pressure range, an increase in CHF is observed with increasing pressure. Nevertheless, one has to notice that this last trend is not so pronounced for narrow as for wide channels. The effect of pressure cannot compensate the decrease in CHF due to the channel gap-size reduction. A previously published correlation for the CHF in such systems is modified so as to take into account the influence of pressure. It is in good agreement with the experimental data since its accuracy is always better than 5%.