The paramagnetic resonance spectrum of the iron oxygen-vacancy (Fe3+-Vo) pair in the tetragonal I4mcm phase of SrTiO3 at 78°K is analyzed in detail. The spectra observed at K band are fully accounted for using the accepted structural properties of the crystal in the low-temperature phase. Whereas in the high-temperature cubic Oh1 phase the Fe3+-VO spectra are axial, in the D4h18-I4mcm phase second-order perturbation terms of an orthorhombic E(T)(Sx2-Sy2) term have to be taken into account with E(T)=1.822(T) cm-1, (T) being the intrinsic rotational (order) parameter of the transition measured in radians. The analytically well understood spectrum justifies its use for the analysis of the structural phase transition observed in SrTiO3 under applied stress and for the investigation of critical phenomena. © 1972 The American Physical Society.