The surface specific analytical method, X-ray photoelectron spectroscopy (XPS), has been used to study the effects of water vapour and CO 2 on the cubic and orthorhombic polymorphs of C 3 A. Significant differences between the two polymorphs were observed in the XPS spectra. Upon exposure to water vapour, both polymorphs produced C 4 AH 13 on their surfaces. Additionally, the sodium-doped o-C 3 A developed NaOH and traces of C 3 AH 6 on its surface. Subsequent carbonation yielded mono carboaluminate on both polymorphs. Large amounts of Na 2 CO 3 also formed on the surface of o-C 3 A as a result of carbonation of NaOH. Furthermore, the extent of carbonation was much more pronounced for o-C 3 A o than for c-C 3 A.