Experiments are presented on the effects of external periodic variation of CO2 concentration on the dissipation and thermodynamic efficiency of photochemistry in whole leaves which have damped oscillations in photochemical activity. The results confirm, in an in vivo system, the possibility of variable efficiency and variable dissipation of chemical reactions, far from equilibrium, obtained by means of external periodic variation of constraints. Leaves of the C-3 species, Arbutus menziesii Pursh., were placed in an atmosphere of supersaturating CO2 concentration (5.0%) and illuminated for a given period with strong actinic light to elicit damped oscillations in the rate of photochemistry. The external concentration of CO2 was then varied between that of normal air and 5.0%. We measured changes in the steady state fluorescence (F’) and the maximum fluorescence during illumination for 1.3 s with saturating light (F’(m)). The experiment was repeated for seven periods and two different phases of CO2 perturbations. From F’(m), and F’ we calculate the changes in Stern-Volmer quenching of the maximum variable fluorescence (SVN) and the quantum yield of photochemistry (phi(PC)). SVN is a measure of the absorbed excitation energy dissipated as heat, and phi(PC) is a measure of the efficiency of photon utilization by photosystem II. When the phase of the perturbation in CO2 concentration is out-of-phase with the onset of illumination, an increase in the average phi(PC) is observed relative to a perturbation of the same period, but in-phase. In addition, for some periods of perturbation, the average SVN for out-of-phase perturbations decreases relative to in-phase perturbations, while at other periods there is no difference between the in- and out-of-phase perturbations. A resonance response in SVN is observed that has a relationship to the phase and period of the perturbation. These results show that the average dissipation and thermodynamic efficiency of photochemistry depend not only on the mean external concentration of CO2 but also oh the phase and period of the external changes in CO2 concentration. We report an unexpected discrepancy between the results of an oxygen evolution measurement and the photochemical yield, phi(PC), obtained from fluorescence measurements in 2.5% and 5.0% CO2. In a companion paper, we present the formulation of a kinetic model for the (damped) oscillations based on ion-gated channels.