Experiments on the incipience of flow boiling in a subcooled well wetting fluid (R12) were performed. The test section was a vertical copper tube of 20 mm inner diameter and 500 mm length. Heat flux and excess wall temperature were measured for upflow and downflow. The mass flux ranged from 200 to 2250 kg/m²s, the subcooling from 2.2 to 10.8 K. For comparison our former experiments with Rll for upflow with 700 to 2200 kg/m²s and subcooling of 3 to 15 K were taken into account. The equations by Bjorge (1979) and Rohsenow (1985) can be simplified. The excess pressure in the "starting nucleus" can be assumed constant.