Monochromatic infrared gas spectrometers cannot identify volatile agents. Such an analyser (Capnomac, Datex ®) was tested while performing two errors : a) erroneous selection of the agent on the analyser, the vaporizer being filled with the correct agent ; b) total or partial filling of the vaporizer (Vapor 19, Dräger ®) with an incorrect agent, the analyser being set for the agent the vaporizer was specified for. Three agents were studied, halothane (H), enflurane (E) and isoflurane (I). Each experiment was made in triplicate, the vaporizer being dried out between each. In case of erroneous selection on the analyser, differences between E and I were very small. When H was erroneously selected, the concentration displayed was six times higher, and when E or I was selected instead of H, the concentration displayed was six times lower than expected. In the 3.3–3.5 m band, H transmittance was six times higher than those of E and I. The agent selection control changes the gain so as to correct for the selected agent (gain for H : 12.0, E : 2.24, I : 1.89). In case of erroneous filling of the vaporizer, the concentration displayed was always different from that expected. When E or I was delivered with an H vaporizer, the analyser being set on H, the concentration displayed was 3 to 9 times higher than the concentration that had been set. On the other hand, when H was delivered with an E or I vaporizer, the analyser being set to E or I, the concentration displayed was 4 to 8 times lower. In the other cases (mixtures of agents in the same vaporizer), concentrations displayed were higher or lower less than twice that which had been set. Therefore, when in clinical practice the concentration displayed is obviously abnormal, the agent delivered should be checked to be the same as that selected on the analyser. Filling of the vaporizer with the wrong agent should then be looked for, especially if the clinical effects are the opposite of those expected from the concentration displayed. Finally, the best means of avoiding such an error consists in the use of an indexed pin safety system.