X-ray photoelectron spectroscopy (XPS) and thermal analysis were used to study a series of lead oxides. X-ray photoelectron spectra suggest that some minium (Pb3O4) samples contain carbonate groups and an excess of oxygen in superficial layers. These samples, if used in paints’ production cause their premature thickening. The combined results provide compositional evidence for the mechanisms occurring at surfaces of underoxidized minium and leading to the formation of hydroxyl and subsequently carbonate species.