Summary:The inhibitory effects of nine nitro and/or bromo‐substituted benzonitrile compounds on the photosynthetic electron flow in isolated chloroplasts and on the in vivo CO2fixation of spinach (Spinacia oteraceaL.) and wheat (Triticum aesticumL. cv. Bezoslaya) were investigated.Bromoxynil and 3‐nitro‐5‐hromo‐4‐hydroxy‐bcnzonitrile were the strongest and equally effective inhibitors of Ihe in vivo CO2fixation of spinach, hut in wheat the nitro‐bromo‐compound is ineffective and 3‐nitro‐benzonitrile is even more inhibitory than bromoxynil. None of the substances affected DCPIPH → methylviologen reduction. In the inhibition of the DCPIP reduction only the 3,5‐disuhstituted 4‐hydroxy‐derivatives were effective. The fact that these compounds affect only the PS II reaction with both H2O and DPC as electron donors suggests a site of inhibition on the reducing side of PS II, between Ihe PS II reaction centre and ihe DCPIP Site. It is suggested that in the inhibition of the DCPIP reduction only steric factors are important and the different electron configuration of the sterieally similar molecules may be involved only in the absorption and translocati