Abstract Spin Lattice Relaxation in the Triplet State of Qainoxaline in Perdeuteronaphthalene and of two Similar Mixed Crystals The spin lattice relaxation in the excited triplet state of three mixed crystals was investigated: Quinoxaline in perdeutero-naphthalene, quinoxaline in naphthalene (X-traps) and quinoxaline in durene. They differ by the depth of their traps, which are shallow (90 cm -1), very shallow (60 cm -1) and deep (6600 cm -1), respectively. In order to identify the relaxation processes and the relaxation mechanism, the experiments were performed in the large magnetic field range be-tween 0.2 T and 5.4 T. By use of a non-resonant optical method and by ESR and ODMR it could be shown that at high fields the direct process (emission of resonant phonons) is the only efficient process up to 4.2 K. At low fields Raman-processes are dominant. Thereby the spin lattice relaxation probability per unit time, w, increases with the ninth power of the temperature in the shallow trap systems and with the fifth power in the deep trap system. By the analysis of the very strong anisotropy of w it could be shown that the efficient relaxation mechanism in the shallow trap systems is a guest-host-interaction modulated by phonons.