The activities of CdI/sub 2/ in NaI and of ThI/sub 4/ in NaI have been determined by using vapor-phase UV absorption spectroscopy. The activity coefficient, ..gamma.., for the CdI/sub 2/-NaI system was found to be constant below 0.05 mole fraction of CdI/sub 2/ and equal to 0.23. For the ThI/sub 4/-NaI system, ..gamma.. was found to be constant below 0.06 mole fraction of ThI/sub 4/ and equal to 0.02. No vapor-phase hetero complexes of CdI/sub 2/ or ThI/sub 4/ with NaI were observed. The effects of vapor-phase dimerization of NaI on its absorption spectrum are discussed and temperature-dependent spectra are presented.