The cleavage of radical anions of substituted alpha-phenoxyacetophenones, X-C6H4COCH2OPh, IIa-k, has been studied in DMF by voltammetric and coulometric techniques. The standard potentials (E degrees) for formation of and rate constants, k, for the cleavage of the radical anions were determined using linear sweep voltammetry, LSV, together with digital simulation and previously reported laser flash photolysis data. The rate constants cover a range of almost eight orders of magnitude (0.4 s(-1) for X = p-MeCO- to 1.3.10(7) s(-1) for X = p-MeO-). The relative driving forces, Delta Delta G degrees(het)(RX.-), for the heterolytic cleavage of the radical anions (to give R-. + X-) were estimated from thermochemical cycles. A combined plot of log(k) versus Delta Delta G(het)degrees(RX.-) for the radical anions of IIa-k and of alpha-aryloxyacetophenones gave a curve with alpha = 0.5 at high driving forces and alpha = 1 at low driving forces, where alpha = partial derivative Delta G(0)(double dagger)/partial derivative Delta G degrees. The plot was analyzed using a model in which reversible cleavage of the radical anions takes place inside the solvent cage followed by (counter)diffusion of the fragments out of the solvent cage. The change in the value of a is interpreted as a change in the rate limiting process from chemical activation (i.e.,, fragmentation) to counterdiffusion. The model allowed the determination of the absolute values of Delta G(het)degrees(RX.-) and the intrinsic barrier, Delta G(0)(double dagger), for the fragmentation of the radical anions (8 +/- 1 kcal mol(-1), 0.35 eV). This leads to an estimate of the homolytic bond dissociation free energy of the C-OPh bond in the unsubstituted alpha-phenoxyacetophenone.