The invention relates to a method and a device (65) for examining a container mouth (4) for defects in a mouth sealing surface (3), wherein a light source (5) serves to emit light beams (22′a,22′b) on to the mouth (4) such that they comprise a direction component impinging tangentially upon the sealing surface (3) and are scattered as they impinge upon a defect in the sealing surface (3), wherein at least some of these scattered light beams (26) are detected by means of a sensor (18′). The tangential direction component is produced by virtue of the fact that in the beam path between the light source (5) and the mouth (4) there is disposed an optical deflecting element (39′) which is transparent at least to light of a certain wavelength and deflects such impinging light beams (22) accordingly. The invention renders it possible to utilize simple means to amplify contrast during the detection of defects in the sealing surface (3).