Apparatus suitable for characterising the molecular species produced when fluorine reacts with a high-temperature metal surface is described and its use illustrated by identifying the species formed when fluorine is passed over heated chromium or iron within a temperature range up to 1000 °C. The species so formed are isolated in argon matrices and characterised by infrared spectroscopy. Metal isotope frequency shifts were used to support identification and the molecular species thus identified included CrF 4 . CrF 3 , CrF 2 , FeF 3 and FeF 2 . For each of the systems studied, it was found that the distribution of reaction products was markedly dependent on temperature. The technique could be generally applicable to the study of any high-temperature metal-fluorine system.