Partial pressures of molecular species over solid PbO were measured in the temperature interval between 890 K and 1100 K using Knudsen-cell mass spectrometry. The assignment of ions is discussed in detail. Structural and vibrational parameters of the gas phase for (Pb n O n ) (n = 1,2,3,4) were calculated by the ab initio molecular orbital method at different approximation levels. Third law enthalpies of dissociation of Pb 2 O 2 and Pb 4 O 4 to monomeric PbO were obtained as (250.6 ± 6) kJ/mol and (835.0 ± 15) kJ/mol, respectively. The possibility of direct measurement of the dissociation pressure of PbO by a mass spectrometer is demonstrated.