Preparation of anaesthesia machines for use by malignant hyperthermia susceptible patients requires purging the machines of halogenated anaesthetic agents. The endpoint of this process is to reach a gas concentration of 5 ppm or less, which has been arbitrarily chosen as the safe limit of exposure to avoid triggering a malignant hyperthermia event. We examined the washout characteristics of sevoflurane and desflurane from the Datex-Ohmeda Aestiva ® Anaesthesia System and Aisys ® Anaesthesia Carestation ® anaesthetic machines. The machines were contaminated for two hours using either sevoflurane 2 vol% or desflurane 6 vol%. At the end of the priming period, the patient breathing circuit and reservoir bag, carbon dioxide absorbent, sampling line and test lung were replaced with uncontaminated components. During the test period, machines were purged using oxygen flows of 10 l/minute. The average time to reach 5 ppm with the Aestiva machines was 51 minutes with sevoflurane and 71 minutes with desflurane. The average time to reach 5 ppm for the Aisys machines was 55 minutes with sevoflurane and 69 minutes with desflurane. All configurations of machines and anaesthetic gases demonstrated a rebound effect in agent concentration above 5 ppm when the fresh gas flow was subsequently reduced from 10 to 2 l/minute. Aestiva and Aisys anaesthetic machines require a prolonged period to adequately purge them of halogenated volatile anaesthetic agent. The rebound effect poses a serious concern, suggesting that after the purging period, fresh gas flows of 10 l/minute should be maintained for the duration of anaesthesia care of the malignant hyperthermia susceptible patient.