The optoacoustic analyzer with a tunable CO2 laser source employed in the present work permits a selective determination of ethylene in trace concentrations higher than 5 ppb (=detection limit for the 10P(14) emission line of the CO2 laser, ν = 949.5 cm-1) and of vinylchloride higher than 42 ppb (= detection limit for the 10P(22) CO2 laser line, ν= 942.4 cm-1). this method covers for both compounds the concentration range corresponding to the hygienic standard. It can be also used for the determination of styrene vapour with concentrations higher than 1.5 ppm.