An optoacoustic (OA) leak detector based on a commercially available CO2 laser is proposed to be used for testing leak-proofness of vacuum vessels. The leak detector with a differential Helmholtz resonator was applied to detection of SF6 admixtures introduced into weak gas flows. The dependence on the flow rate of a gas sample in the detector was analyzed. The minimum detectable flow rate was estimated to be ~ 1 10-9 Pa m3/s, which is comparable with the sensitivity of commercial He leak detectors.