The results of an experimental investigation of pool boiling of water at subatmospheric pressures from small horizontal heat sources are reported. The heat sources are upward-facing copper surfaces submerged in a laterally-confined, finite pool of liquid. The saturated pool boiling heat transfer characteristics and the critical heat flux (CHF) condition were determined in the experiments. Low pressure boiling of saturated water provides a means of removing heat at high heat flux levels while maintaining low surface temperatures. However, at heat flux levels up to about 60 2 W/cm the frequency of bubble departure at low pressure is much less than the frequency of bubble departure at higher pressure (atmospheric). With low pressure boiling, only one or two very large bubbles form cyclically on the small heated surface, during the boiling process. This intermittent process may result in large, undesirable temperature oscillations at the heated surface for low pressure boiling. High-frequ...