Using microwave induced phosphorescence techniques we determined the rate constants for populating and decay of the lowest triplet state of tetramethylpyrazine (TMP) in a durene crystal. The dynamic behaviour of the spin statesTy′andTz′corresponding to the in-plane spin axes (principal axes of the dipolar coupling tensor)y′andz′proves to be identical :ky′r=kz′r(radiative decay),ky′=kz′(absolute decay rates) andPy′=Pz′(populating rates). This result supports the suggestion of de Grootet al., based on E.S.R. experiments, that for the phosphorescent state of TMP in the durene host the in-plane spin axesy′andz′have undergone an appreciable (≈45°) rotation with respect to the in-plane molecular axesyandz.