K-β-Ga2O3, Na-β-Ga2O3, and Na-β″-Ga2O3 are alkali fast ion conductors which crystallize with the β and β″ Al2O3 structures. These compounds exhibit an excess of alkali ions which must be compensated by negatively charged defects to maintain charge neutrality. Single crystal X-ray diffraction has been used to elucidate the defect structures. Compensation in K-β-Ga2O3 occurs through the presence of interstitial oxygen, as observed in Na-β-Al2O3. Compensation in Na-β-Ga2O3 occurs through two processes. One third of the excess sodium is compensated by interstitial oxygen. The remainder is compensated by immobile sodium, which has replaced gallium on tetrahedral sites. Compensation in Na-β″-Ga2O3 is achieved exclusively by immobile sodium. The observed site distributions in K-β-Ga2O3 and Na-β-Ga2O3 do not fit the WGC or Wolf models, but can be accounted for in terms of a new type of cluster. © 1981.