A photoacoustic cell assembly is described that is permeable to CO2 and other gases but not water vapor. As a replacement for the usually employed solid cover, this cell uses a cover containing a small fritted glass disk that holds a small piece of 6.4 μm Teflon film against the sample.With the above arrangement it was possible to increase the rate of O2 evolution measured photoacoustically about 3 times in Zea mays leaves and about 1.7 times in Phaseolus vulgaris leaves upon adding CO2 to the gas stream. The extent of energy storage was also enhanced with supplemental CO2 in Zea and Ulva but less so in Phaseolus. The maximum improvements of photosynthetic activities were obtained when the gas stream contained 2.5-5% CO2. These high concentrations were presumably necessary as the result of a high resistance to diffusion through the gas-permeable cover.