The kinetics of the fast unimolecular fragmentation of alpha-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 x 10(5) s(-1) to 2.8 x 10(8) 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 10(5) s(-1) are studied by linear sweep voltammetry.