The defect complexes formed when the univalent metal ions Na+ or K+ are incorporated in CaF2, SrF2, or BaF2 have been investigated by the technique of thermal depolarization. Only a single depolarization event is found in CaF2:Na+ and SrF2:Na+ and this is ascribed to nearest neighbour Nas+Fv− complexes (s = substitutional ion, v = vacancy). Evidence for this kind of complex is also observed in BaF2:K+ and a second depolarization peak at 138 K in these crystals is attributed to an M+ impurity.