An expansion cloud chamber has been developed which operates at a pressure before the expansion of about 45 millimeters of mercury. Since evaporation is rapid and the temperature very low during the sensitive time of the chamber, the density of the gas is higher after the expansion than before. The composition of the gas before the expansion is approximately 40 percent water vapor and 60 percent permanent gas. When the chamber is operated with oxygen or argon, the stopping power of the gas during the sensitive time is 7.6 percent that of NTP air. When it is operated with helium, the stopping power is 5.7 percent that of NTP air.