One of the ideal theoretical devices for determining flame velocities is the constant-pressure adiabatic bomb. In practice this has been realized in the form of a soap bubble filled with a premixed gas-oxidant combination, centrally ignited by a spark, and observed by high-speed photography. However, the aqueous soap film imposed certain restrictions, in that moisture content was not controllable, and that the film itself might have taken some part in the combustion reaction. The present paper reports (a) on a search for a substitute material for the bomb, and (b) the development of a technique in which a very thin transparent rubber balloon was used. The new method was then used to determine flame velocities in carbon monoxide-oxygen mixtures saturated with water vapor. These velocities were found to increase with increase of initial mixture temperature.