The Bureau of Mines investigated the thermodynamic properties of bi2o3 (bismuth sesquioxide) and sNO2 (stannic oxide). Equilibrium oxygen pressures of the bi-bi2o3-o2 systems were measured by a high- temperature electromotive force (emf) method using a solid electrolyte tube of stabilized zro2 (zirconia). The emf measurements yielded standard Gibbs energies of formation of bi2o3 and sNO2 from the elements. The enthalpy and temperatures for the a to g phase transformation of bi2o3 were determined. These data for bi203 and sNO2 were compared with previously reported measurements, and discrepancies were discussed. Standard enthalpies of formation of bi2o3 and sNO2 were derived by combining results from this investigation with auxiliary data from the literature.