CIDNP spectroscopy (measurements of chemically induced dynamic nuclear polarization) is applied to the photoreaction of methionine with 4-carboxybenzophenone in D2O at varying pH (5.8,..., 12.2). By using the polarization pattern of the regenerated amino acid, the interconversion of the different forms of the methionine radical cation (open-chain protonated, open-chain deprotonated, and cyclic, with a two-center-three-electron bond between sulfur and nitrogen) is studied. The change of the CIDNP pattern with pH is not due to a protonation preequilibrium but is a rate phenomenon. To extract rate constants from the pH dependence of the polarization pattern, the theory of pair substitution in CIDNP is extended to cover reversible reactions with arbitrary equilibrium constants. This problem is treated with the Freed-Pedersen reencounter formalism. Spin dynamics and radical pair dynamics are separated by the assumption of an exchange volume. General expressions for the spin-dependent recombination probabilities in the strong-exchange limit are derived, as well as solutions for a specific diffusional model (Noyes＇ model); the latter are used to fit the experimental data. It is shown that neither the assumption of slow (on the CIDNP time scale) protonation/deprotonation nor that of pi-I-independent reaction rates can explain the observed effects. When the rate of the backward reaction is proportional to [H+], which follows from the assumption that cyclization of the deprotonated open-chain radical cation is fast on the CIDNP time scale, a very good fit can be reached. The pK value for deprotonation concomitant with cyclization is found to be 8.15. By comparison with model compounds it is estimated that the equilibrium constant for cyclization of the deprotonated radical cation is about 2.5.