Single electron transfer (SET) takes place from tributylphosphine (1) to 1-methyl-1'-alkyl-4,4'-bipyridinium salts (viologens; 2) in acetonitrile-methanol. Significant kinetic deuterium isotope effect has been observed with respect to hydrogens of a methyl (or an ethyl) substituent in 2. The results are interpreted in terms of SET occurring within a tight encounter complex formed between 1 and 2.