The vaporization behavior of the Al-O system has been studied on numerous occasions but significant uncertainties remain. The origin of this uncertainty must be understood before Al-O vaporization behavior can be accurately determined. The condensation of Al2O3 and clogging of the effusion orifice is a difficult problem for the Knudsen effusion technique that influences the measured vaporization behavior but has only received limited attention. This study reconsiders this behavior in detail. A new theory for Al2O3 condensation is proposed together with procedures that will improve the measured thermodynamic properties of Al-O vaporization. (C) 2005 Elsevier Ltd. All rights reserved.