Temperature dependences of electron spin relaxation times of a phenoxyl radical derivative (2,6-di-tert-butyl-4-hydroxymethyI phenoxyl) dispersed in a diamagnetic matrix have been studied by a pulsed ESR technique. With these temperature variations from room temperature to about 10 K, the molecular environmental effects to electron spin relaxations have been investigated. The relaxation times indicated minimums at 200 and 90 K. It may be concluded that the relaxation times vary depending on the motional fluctuation of substituted molecular groups on the pi-conjugated framework. The obtained rotational activation energies of methyl or tert-butyl groups were very small. It is considered that the sample lattice is much less rigid for the hindered motions. In addition, the activated barrier of methyl or tert-butyl group rotations is seemed to be consistent with the volumes of these substituted molecular groups. (C) 2004 Published by Elsevier B.V.