A crystallographic and thermochemical study of (\mathrm{CaUO}{4-x},\left(\mathrm{Ca}{0.5} \mathrm{Sr}{0.5}\right) \mathrm{UO}{4-x}), and (\alpha-\mathrm{SrUO}{4-x}) with (x) values ranging from nearly 0 to 0.5 was carried out. As the crystal radius of the divalent cations increases, a(rhomb) of neariy stoichiotnetric compounds increases, accompantied by an increase in the cell volume, whereas (\alpha() rhomb) decreases. With nonstoichiometry, the lattice parameters of (\left(\mathrm{Ca}{0.5} \mathrm{Sr}{0 . j}\right) \mathrm{UO}{4-x}) and (\alpha)-SrUO S-x discontinuously change at (4-x=3.79) and 3.77 , respectively, although each compound is a single phase in the entire range from (4-x=4.00) to 3.50 . For (\mathrm{CaUO}{4-x}), the products were two-phase mixtures between (4-x=3.98) and 3.70 . These crystal structures are discussed taking into account the lattice parameters and interatomic distances. In the nonstoichiometric crystals, oxygen vacancies exist on the (\mathrm{O}{11}) sites but not on the (\mathrm{O}{1}) sites. The enthalpy of formation of (\alpha)-SrUO ({ }{4-x}) was measured for several specimens with different (x) values by solution calorimetry, and was expressed in a second-order polynomial of (x) by least-squares calculation. This quantity and its derivative, i.e., partial molar enthalpy of oxygen, suggest the change of defect species with (x) and the existence of repulsive vacancy-vacancy interaction. 1993 Academic Press. Inc.