In the lowest triplet states of naphthalene, pyrene and chrysene certain ring carbon atoms should carry a negative spin density because the exchange field of the excited electron and ‘hole’ polarizes the spins of the remaining pairedπelectrons. Extended Hartree-Fock and configurational mixing theories give almost equivalent descriptions, and both predict that the negative spin densities in the triplet state are nearly the same as in the positive or negative ion radicals of the hydrocarbon. Similar exchange effects produce antiparallel correlation between the spins of atoms with opposite spin densities. These will reduce the zero-field splittingD, but probably not be strong enough to change its sign.