E.s.r. study of triplet state aryl cations generated photochemically (hν at 77 K) by the process ArN2+BF4–→ Ar++ N2+ BF4 indicates that the lowering of the triplet state energy level of Ar+(relative to the almost isoenergetic singlet level) to become the ground state, is promoted by the presence of electron-releasing groups, such as NR2, OR, SR, and SAr; the stabilising effect of a given substituent is positionally dependent, viz. 4 > 2 3 substitution, whilst for a given site of substitution the stabilisation effect is NR2 > OR SR SAr. The magnitude of the D parameter reflects both this sequence and the presence of other electron-withdrawing substituents. It is concluded that ab initio calculations cannot deal adequately with the subtlety of the effects promoting stabilisation of triplet Ar+.