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The EPR spectra of Mo3* ions in corundum a t v = 9.25 G H z and T = 77 OK were investigated. It is shown, that Mo3+ ions isomorphously replace A13+ ions in the octahedral coordination. Due to the large zero-field splitting, only the -1/2 -+l/2 transition is observed. The zero-field splitting constant 2 0 5 (-49 * 18) GHz, the spectroscopic splitting factor 911 = 1.968 & 0.001, 91 = 1.98 & 0.01, and the hyperfine splitting constant All = (119 fl) MHz, A 1 = (121 5 2) MHz are determined. It is shown that in the case of large zero-field splitting 2 0 >> v the magnitude of 2 0 can be determined by measurements a t a single frequency, using the value of geff a t 3 5 ' 2 ' (case 0 = 2/3). The angular dependence of the hyperfine structure splitting due to the large zero-field splitting is observed and good agreement with values calculated in first order of perturbation theory is obtained. The spin-lattice relaxation times of M03+ ions in corundum are estimated from the magnitude of the broadening of the absorption curves with increasing temperature.I n the temperature range of 77 to 500 O K the spin-lattice relaxation rate is z l l -T2.5.

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