CIDNP (chemically induced dynamic nuclear polarization) effects in the sensitized photoreactions of triethylamine DH (sensitizers A, 9,10-anthraquinone and 2,7-dinitrofluorenone) are investigated in a series of aprotic solvents of widely varying relative permittivity epsilon, including mixtures of acetonitrile and chloroform. The dependence of the polarization pattern, that is, the relative polarization intensities of the olefinic alpha- and beta-protons in the reaction product N,N-diethylvinylamine, on epsilon is studied quantitatively be evaluating the polarization ratio r of these protons. The factor responsible for the change of the polarization pattern is the rate, relative to the kinetic window of CIDNP, of in-cage deprotonation of the triethylamine radical cation by the sensitizer radical anion (rate constant k(dep)), which converts the initially formed radical ion pair (A(.-)DH(.+))over-bar into a neutral radical pair (AH(.)D(.))over-bar. A theory of pair substitution taking place solely during encounters of the radicals is presented. Based on this, an expression for the dependence of r on k(dep) is derived, which in turn allows the extraction of k(dep) from the experimentally observed polarization ratio. The accessible range of k(dep) in these systems is from about 10(8) to about 10(10) M-1 S-1.