The electron spin resonance of the minority defect Fe4+-V0 has been observed in SrTiO3. It is the first pair center identified with a two-fold positive charge with respect to the lattice. For Fe4+ it occurs in the unusual high spin S = 2 state. Optically-induced conversion to the neighbor Fe3+-V0 charge state served to identify the acceptor transfer bands, showing the potentiality of the conversion-rate method. © 1975.