1. The second-order rate constants of inhibition, ka, of acetylcholinesterase were measured at pH values between 5·5 and 10·5 for two esters of phosphorus acids and five esters of carbamic acids. Two of the carbamates and one of the phosphates contained a quaternary nitrogen group. 2. For the three positively charged compounds the ka–pH plots are bell-shaped, with a pH optimum between 7·5 and 9·0. The changes in ka above and below the optimum pH fit theoretical curves for the dissociation of groups on the protein of pK 6·2 and 10·25. 3. For the uncharged compounds, the ka–pH plot on the alkaline side is identical with the one obtained for charged inhibitors. On the acid side they do not fit such a curve and the ka for two of the carbamates is independent of pH changes between 5·5 and 8·0. 4. The first-order rate constants, k+3, for spontaneous reactivation were measured at pH values between 5·0 and 11·0 for N-methylcarbamoylated, NN-dimethylcarbamoylated and di-(2-chloroeth)phosphorylated cholinesterase. For all three derivatives the k+3–pH plots are bell-shaped, with a pH optimum between 8·0 and 8·5. The changes in k+3 above and below the optimum fit theoretical curves for the dissociation of groups of pK 6·9 and 9·8. 5. The relevance of these results to binding, acylation and deacylation of both inhibitors and substrates is discussed.
