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Recent EPR studies of the 4d1 configuration ion Zr3+ in the tetragonal-symmetry orthophosphate hosts LuPO4, YPO4, and ScPO4, have led to additional investigations of d1 configuration ions associated with unusual valence states of other elements. Single crystals of LuPO4, YPO4, and ScPO4 doped with hafnium were grown and the electron paramagnetic resonance (EPR) spectrum of the 5d1 ion Hf 3+ was observed in all three cases. EPR signals due to trivalent hafnium were only observed, however, when the doped samples were gamma irradiated at 77 K and maintained at 77 K during the EPR observations. Since there are two odd isotopes of hafnium with different nuclear spins and different natural abundances, identification of the spectrum of Hf 3+ was straightforward. Since Hf 3+ represents such an unusual valence for hafnium in the solid state, however, this identification was unequivocally verified by preparing crystals doped with isotopically enriched 179Hf. The spectroscopic splitting factors, g|| and g⊥, and the parallel magnetic hyperfine constants A|| of 177Hf and 179Hf were obtained for Hf 3+ in all three hosts. A determination of the hyperfine parameters with the applied magnetic field oriented in the perpendicular direction was not carried out due to quadrupole effects. As in the case of Zr3+, the observed electronic g values for Hf 3+ could not be accounted for by the standard second-order equations which included the spin-orbit-induced admixture of some other excited orbital levels of the d1 configuration into the ground orbital level. The ratio of the nuclear magnetic moments 177μ/179μ, was determined to be 1.2361(4) and 1.2362(3) for hafnium in ScPO4 and LuPO4, respectively. [ABSTRACT FROM AUTHOR]