A measurement system and technique have been developed for the evaluation of interactions between anaesthetic agent vapours and carrier gases. The refractivities of mixtures of anaesthetic agent and carrier gas have been compared to values derived from the addition of the absolute refractivities of the individual components. These results show that there is no significant interaction between the anaesthetic agent and the carrier gas at the +or-0.06% level of uncertainty. Results for the anaesthetic agents isoflurane, sevoflurane, enflurane, halothane, chloroform and desflurane and the carrier gases nitrogen, oxygen and dry air are presented.