Sevoflurane is degraded by soda lime to a vinyl ether commonly referred to as compound A. We measured the concentration of compound A in the circle breathing system of 31 patients receiving sevoflurane anaesthesia. Inspiratory and expiratory gas samples were analysed using gas chromatography and flame ionisation detection. The end-tidal sevoflurane concentration and soda lime temperature were recorded. The peak compound A concentration ranged between 10 to 32 ppm in the inspiratory limb and 7 to 26 ppm in the expiratory limb. There was a positive correlation between the peak compound A concentration and the end-tidal sevoflurane concentration (r2 = 0.545, p < 0.0001) and the soda lime temperature (r2 = 0.301, p = 0.0014). We conclude that the end-tidal concentration of sevoflurane and the temperature of the soda lime are important variables in determining concentration of compound A in a circle system.