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The relative stabilization of 10 4-monoand 24 4,4'-disubstituted triphenylmethyl radicals 1 has been measured by recording the degree of dissociation of the corresponding quinonoid dimers 2 by means of ESR. The following substituents or combinations of two of them have been used: H, CF3, r-Bu, OMe, OPh, CN, COPh, COMe, Ph, SMe, and N02. Both donors and acceptors enhance the stability in the ground state of the radicals, which is evaluated in terms of ' values and a Hammett-like equation. Two donors act additively, as do two acceptors. No specific synergism of a donor with an acceptor (capto-dative stabilization) has been found. Most efficient for the relative stabilization are the electroneutral substituents Ph and SMe. Most of the ESR spectra of these trityls are new. An exact assignment became possible via the corresponding ENDOR spectra, which are listed in detail. Many of the substituted trityls including dimers and precursors have been prepared