We describe the existence of six thermodynamically stable phases on the binary line Bi2O3- SeO2 at room temperature: Bi12SeO20, Bi10Se2O19, Bi16Se5O34, Bi2SeO5, Bi2Se3O9 and Bi2Se4O11. At higher temperature we optained evidence for Bi20Se3O36 and Bi4SeO8. The decomposition pressures were measured for all phases in a membrane zero manometer and the enthalpy of formation and the standard entropy was determined. The phase barogram and phase diagram followed from total pressure measurements and differential thermoanalysis. The standard enthalpies of formation of all phases were also derived from solution calorimetry. The necessary data of transitions on molar enthalpies were obtained by DSC- and Cp-measurements.