Optical absorption spectroscopy and electron spin resonance (ESR) techniques were used to study the site occupations for Nd 3+ in β″ alumina. In partially exchanged compositions (3.2 × 10 20 Nd/cm 3) the Nd 3+ occupy midoxygen sites primarily. There is some population of the tetrahedral site; however, the concentration is too low to be clearly observed by ESR. Crystals which were completely exchanged (∼1.8 × 10 21 Nd/cm 3) exhibited rather different behavior. The spectroscopy results for these materials indicate that there is a combination of both isolated Nd 3+ ions and discrete pairs.