The activation volume for the RIV relaxation, that is for the reorientation of a dimer, has been determined for calcium fluoride crystals doped with europium, gadolinium, terbium or dysprosium. The results range from 1.05+or-0.15 cm3 mol-1 for europium to 0.3+or-0.15 cm3 mol-1 for dysprosium. These values are appreciably less than the activation volumes for the motion of bound interstitials or bound or free vacancies. In contrast, the activation enthalpies for the reorientation of the dimer are rather greater than those for interstitials or vacancies. These results, together with the results of a computer simulation, show that a 2-2-2 cluster cannot be responsible for the relaxation. It is pointed out that a gettered 2-2-2 cluster cannot be ruled out and, in fact, contains several of the characteristics which are consistent with the experimental results of the present work.