1. The stoicheiometry of the photo-oxidation of succinate by chromatophores has been investigated with [2,3-(14)C(2)]succinate. It was found that there is a stoicheiometric relationship between the amount of succinate oxidized and the NAD reduced, and that fumarate is the only product of succinate oxidation. 2. The possibility of a direct hydrogen transfer from succinate to NAD in this reaction was investigated with tritiated substrates. With tritiated succinate less than 3% of the activity expected if direct hydrogen transfer occurred was recovered in the NADH(2), and this was due to contamination with the substrate. In experiments with tritiated water, NADH(2) was labelled, and had half the specific activity of the water, as expected if water was the source of protons. It was also found that chromatophores catalyse an exchange reaction between NADH(2) and water. 3. It is concluded that the exchange reaction makes it impossible to interpret these results as indicating either a hydrogen-transfer or an electron-transfer mechanism for the photoreduction reaction.