The glow curve of chloroplasts excited by continuous light of high intensity (500 W · m−2) at pH 7.5 during cooling from +2 to −80°C consisted of seven bands appearing at about −30°C (TL−30), −15°C (TL−10), +10°C (TL+10), +30°C (TL+30), +50°C (TL+50), +65°C (TL+65) and +85°C (TL+80), in which TL stands for thermoluminescence. In the pH range from 5.5 to 9.0 the peak positions of the TL−30, TL−10, TL+50, TL+65 and TL+80 bands were independent of pH. On the other hand the peak positions of the TL+10 and TL+30 bands were gradually shifted from +25 to −5°C and from +20 to +40°C, respectively, as the pH was decreased from 9.0 to 5.5. The same pH-induced shift (from +25 to −5°C) was observed for the TL+10 band when electron transport was inhibited by DCMU. In dinoseb-treated chloroplasts the peak position of the main thermoluminescence band also exhibited pH dependency, and shifted from +20 to −20°C upon lowering the pH from 9.0 to 5.5. After the water-splitting system had been inactivated by Tris or NH2OH treatment no pH-induced shifts were observed in the peak positions of the thermoluminescence bands of DCMU and dinoseb-treated chloroplasts. The results suggest that the effect of pH on the thermoluminescence of untreated and inhibitor-treated chloroplasts is associated with protonation/deprotonation reactions occurring at the donor and acceptor sides of Photosystem II during the S1 → S2 transition of the water-splitting system.