Abstract 1. The inhibitory action of tetraphenylboron, a lipid-soluble anion, on the proton uptake, the photophosphorylation and the light-induced quenching of the fluorescence of 9-aminoacridine by spinach chloroplasts was studied. 2. The inhibition of the three processes by tetraphenylboron was transient; the proton uptake was affected to a much smaller extent than either the photophosphorylation or the fluorescence quenching. 3. The inhibitory effects of tetraphenylboron on the proton uptake and the fluorescence quenching of 9-aminoacridine were qualitatively the same in CF 1-depleted chloroplasts, that were recoupled with N, N′-dicyclohexylcarbodiimide (DCCD). 4. The reversal of the fluorescence quenching of 9-aminoacridine upon addition of tetraphenylboron in the light was found to be very fast, being completed within the response time of the apparatus. 5. The presence of tetraalkylammonium salts in the incubation medium prevented the inhibitory effect of tetraphenylboron. 6. Tetraphenylboron disappeared from the chloroplast suspension in a light-dependent irreversible way; in the dark no ‘ptake’ of tetraphenylboron could be detected. 7. The effects of tetraphenylboron may be explained by the presence of groups with a high affinity for tetraphenylboron in the membrane; these groups become protonated upon illumination of the chloroplasts.