X band microwave-optical double resonance studies of the metastable triplet states of naphthalene-d8and quinoxalene are presented and the amplitudes and linebreadths of the signals examined. It is noted that the amplitudes of the lines are consistently an order of magnitude or more smaller than expected on the basis of a simple theory; the discrepancy is shown to arise in part from the long triplet spin-lattice relaxation times which prevailed under the conditions of the measurements.Kband microwave optical double resonance results for quinoxalene support this analysis and indicate that the double resonance method can be extended to permit study of the ESR of triplet systems of photochemical interest which are too short-lived to be observable by conventional ESR methods. A successful application of the double resonance method to triplet cyclopentanone is reported.