1. The transport of electrons from H 2O to lipophilic oxidants such as oxidized p-phenylenediamines and 2,5-dimethylquinone, when observed in the presence of the plastoquinone antagonist dibromothymoquinone, has a pH optimum of approximately 7.5 and is independent of the presence or absence of ADP and phosphate. Nevertheless the electron transport supports phosphorylation with an efficiency ( P e 2 ) of 0.3–0.4 and this efficiency is practically pH independent. A reversible proton uptake is associated with the electron transport. The energy coupling site responsible for the phosphorylation, which must be before plastoquinone, we have designated Site II, while the well-known rate-determining coupling site after plastoquinone and before cytochrome f is referred to as Site I. 2. The transport of electrons from reduced 2,6-dichlorophenolindophenol (DCIP) to methylviologen, when observed in the presence of the Photosystem II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, is remarkably similar in most respects to the overall Hill reaction ( e.g. H 2O→methylviologen). The rate of electron flow is markedly stimulated by ADP and phosphate. Electron transport and phosphorylation have the same pH optimum of about 8.5. The P e 2 ratio is also strongly pH dependent, showing a similar pH optimum of 8.0–8.5. However, the absolute value of the P e 2 ratio observed for the partial reaction reduced DCIP→methylviologen is lower than the P e 2 ratio observed for the overall reaction H 2O→methylviologen at all pH values. The maximum P e 2 value observed for the reduced DCIP→methylviologen raction is 0.5–0.6 at pH 8.0–8.5 while the maximum value for the H 2O→methylviologen reaction under the same conditions is about 1.1. 3. When the P e 2 ratios for the two partial reactions (H 2O→dimethylquinone and reduced DCIP→methylviologen) are added together at all pH values from 6 to 9, the resulting curve is very close to the P e 2 -pH profile experimentally obtained for the overall Hill reaction H 2O→methylviologen. It seems probable, therefore, that the transport of electrons from reduced DCIP to methylviologen utilizes only the rate-determining coupling Site I while the overall transport of electrons from H 2O to methylviologen utilizes both Site I and Site II.