The effect of temperature on the carboxylation activity of intact spinach chloroplasts was investigated in the “carbon dioxide limited state”. The Arrhenius plot of the carboxylation activity shows a break in the respons at about 13 °C. Below this critical point, the carboxylation activity is independent of temperature, apparent Q 10 is 1 and the activation energy is null. Above 13 °C, the carboxylation activity increases with temperature with a Q 10 of 2.5 and an activation energy of 15.3 Kcal/mole. Hence, there is a shift of the reaction by which CO 2 limits the rate of chloroplast carboxylation from a physical type (below 13 °C) to a biochemical type (above 13 °C). From comparison with the temperature effect on biological membranes and their activities reported in the literature, it is assumed that the differential respons to increasing temperature of the chloropoast carboxylation in “CO 2 limited state” would be indicative that the limiting step of CO 2 flux into the chloroplast would be at the level of the envelope, and that the increase in activation energy at the critical temperature would result from a temperature induced phase change of the lipids of the envelope membranes.