In this study, prediction of the thermal decomposition pathway of the volatile liquid anaesthetics such as halothane, enflurane and isoflurane in contact with various metal/metal oxides at elevated temperatures has been deduced by evaluating the mass spectrometric fragmentation pattern of each anaesthetic observed in the ionisation process. In the light of the molecular structures and fragmentation components, it was believed that the thermal decomposition pathway of each anaesthetic on metal/metal oxide surface, particularly at higher temperatures, is similar to the ionisation mechanism occurring in the mass spectrometer ionisation process with minor differences for each molecule. The ionisation clusters for each anaesthetic molecule show the most likely fragment and radicals formed in the mass spectrometric ionisation process by considering the isotopic effect. From these clusters, thermal decomposition pathway of the liquid anaesthetics and formation mechanisms of the major halogenated thermal decomposition products have been predicted. It was concluded that the ionisation and thermal decomposition pathway resembles each other, but are not completely similar. (C) 2003 Elsevier Science B.V. All rights reserved.