The kinetics of the fast unimolecular fragmentation of α-aryloxyacetophenone radical anions in DMF has been analyzed by using a combination of linear sweep voltammetry and independently determined standard potentials or rate constants. The cleavage rates are in the range 3.8 × 10⁵ s^-1 to 2.8 × 10⁸ s^-1. The mechanism changes from a predominantly DISP mechanism to a predominantly ECE mechanism at fast cleavage rates. It was found that the effect of the heterogeneous electron transfer cannot be neglected when reactions with rate constants above 10⁵ s^-1 are studied by linear sweep voltammetry.