Аннотация:On the pseudobinary section YbOCl/SeO2 four thermodynamically stable compounds exist in accordance with the thermal decomposition and the phase analysis of defined powder mixtures: YbOCl (.) 3 SeO2 = YbSe3O7Cl, YbOCl (.) 2 SeO2 = YbSe2O5Cl and YbOCl (.) SeO2 = YbSeO3Cl as well as the YbOCl-rich phase Yb2SeO4Cl2 (2 YbOCl (.) SeO2). The phase barogram and the phase diagram of the pseudobinary section YbOCl/SeO2 were set up by total pressure measurements and DTA. From the temperature dependence of the decomposition pressures the enthalpies of formation and the standard entropies of the phases were deduced. In addition, the enthalpies of formation of the new compounds were determined from the enthalpies of solution of YbOCl, SeO2 and the quaternary phases.