Effusion cell beams containing the, gaseous species HPO, HPO2, HPO3, and POBr along with other products were generated by the reaction of H2O(g) or Br-2(g) with Ca2P2O7(s). The species were identified by mass spectrometry, and were characterized thermochemically from the study of several reaction equilibria. Reaction enthalpies ad standard enthalpies of formation were derived from third law analysis of the equilibrium data, yielding the Delta(f)H degrees(298), values HPO, -34.2; HPO2, -110.6; HPO3, -168.8; and POBr, -53.5, all in kcal mol(-1.) Our new result for POBr is in serious disagreement with a value reported in,the literature; there are no previous reports;ts on the other species studied here. A preliminary Delta(f)H degrees(298) value of -60.2 kcal mol(-1) was also obtained for POCl(g), this time in reasonable accord with a literature value. The new thermochemical data yield calculated compositions in agreement with measured values for some lamp gas mixtures.