An equation is derived for converting volume changes observed in constant-pressure respirometers to total amounts of gas exchanged, expressed as volumes at standard conditions. If the amounts of dissolved gases are not considered, errors in O 2 measurements are within the tolerances usually accepted for manometry but large errors occur in CO 2 measurements. Under some conditions, the changes in the partial pressures of nonmetabolized gases, which occur during an experiment, can also introduce a significant error. A practical simplification of the complete equation has been shown theoretically and experimentally to correct for these factors with sufficient accuraey for all practicable experimental conditions.