In memoriam Professor Gerhard LehmannElectron paramagnetic resonance spectra of tetravalent terbium ions have been observed in host crystals of ZrSi0 4 , HfSi0 4 , ThSi0 4 , ThGe0 4 (zircon structure) and ZrGe0 4 , HfGe0 4 (scheelite structure) at X band and Q band and various temperatures. The spin Hamiltonian parameters have been determined from the rotational diagrams; the zero field splittings turned out to be extremely large, so that complex spectra far from the strong magnetic field limit had to be analyzed. In all the compounds Tb 4+ substitutes for the tetragonal metal lattice sites. The incorporation of terbium, the origin of the large second-order parameters and comparisons with the spectra of trivalent gadolinium in similar systems are discussed.