Photoionization of N, N, N', N'-tetramethyl-p-phenylenediamine ( TMPD) in alcoholic solution produces the radical ion pair [TMPD center dot+ e(-)]. However, the identity of the negatively charged counter-radical formed by photolysis of TMPD in DMSO (dimethylsulphoxide)/toluene mixtures, for which unusually large effects of weak applied magnetic fields have been observed, is unclear. Modulated MARY (Magnetically Affected Reaction Yield) experiments on solutions containing different isotopomers of TMPD, DMSO and toluene show that the counter-radical is likely to be the solvated electron. This result supports the idea that large effects of weak fields on radical recombination yields can be expected for radical pairs in which the electron-nuclear hyperfine interactions are concentrated in one of the radicals, rather than being distributed more evenly between the two radicals.