The translational diffusion coefficients (D) of N,N,N ,N -tetramethyl-p-phenylenediamine (TMPD) and its cation radical were measured by the transient grating method. The D value of the free cation radical is about twofold smaller than that of the neutral molecule. The D values of both species were calculated on the basis of the Stokes-Einstein (SE) relation, the Spernol-Wirtz modification of the SE relation and the Hubbart-Onsager theory. The calculations and comparisons with previously determined D values of neutral radicals suggest that, contrary to the intuitive picture of the diffusion of charged species, it may be the unpaired electron and not the charge that mainly governs diffusion.