The inception of bubbles for forced convection flow over a heated plate in a pressurized channel and their consequent mutual interaction in the formation of a “bubble boundary layer” was investigated. A high pressure closed loop was designed to permit photographic study of boiling over the heated plate and to provide for the measurements of temperature profiles in the boiling fluid. The plate, which is 0.50 in., by 0.48 in., was vertically oriented in a rectangular channel 0.50 in., by 0.46 in., in cross section. Photographs and temperature profile measurements were obtained for forced convection boiling of distilled and degassed water at pressures of 200 to 1000 psia, velocities of 1 to 6 ft/sec, subcooling of 50–300 deg F. Experimental data on the bubble boundary-layer thickness were correlated to within ±20 percent by a single equation covering the range of variables investigated.