Abstract 1. The midpoint potential of cytochrome b 6 in freshly prepared coupled chloroplasts is approximately +5 mV and the slope of the titration curve is characteristic of a two-electron transition. Subsequent titrations performed one-half hour or more after chloroplast preparation show a slight negative shift of the midpoint potential and a one-electron slope. 2. Titrations of cytochrome b 6 in the presence of carbonylcyanide- p-trifluoromethoxyphenylhydrazone (FCCP) or NH 4Cl show the midpoint potential of approximately half the b 6 complement negatively shifted to about −140 mV. 3. Brief exposure of the chloroplasts to NH 4Cl and actinic illumination under typical uncoupling conditions with controlled potential conditions causes a shift in oxidation state consistent with a lowering of the average midpoint potential to about −100 mV. It is concluded that the midpoint potential of at least half the cytochrome b 6 undergoes a negatively directed shift of 100–150 mV under uncoupling conditions in the presence of NH 4Cl and FCCP.