Isotropic features with hyperfine structures in electron paramagnetic resonance spectra of SrS powder phosphors, which reflect the abundance of the natural Cr isotopes, are attributed to the isolated Cr impurity. The isotropy, which is typical for a4A2ground state of a3d3ion in an octahedral crystal field, and thegvalue of 1.9795 verify that Cr is observed in its charged donor stateCr3+.The observation of a broad photoluminescence band with a maximum at 900 nm, which can be attributed to an internalCr3+transition, supports the identification of the impurity asCr3+.©1998 American Institute of Physics.