An experimental investigation has been performed on critical heat flux (CHF) in subcooled upflow boiling with a uniformly heated vertical tube, using R12 as a coolant, to evaluate the dependence of the CHF location on the variation of the thermal hydraulic parameters affecting the thermal crisis, such as mass flux and heat flux. The test section consists of a stainless steel tube, 7.57 mm i.d., 2.1 m long, uniformly heated using d.c. current. The research showed a different behaviour of the CHF location following mass flux and heat flux variations in the low and in the high quality regions. The sensibility of the CHF location upon the variation of a thermal hydraulic parameter, such as mass flux or heat flux, decreases as the critical quality decreases, and with increasing of the mass flux. At very low critical qualities the CHF location shows a relative insensibility upon large variations of thermal hydraulic parameters. A weak influence of pressure has also been detected.