An attenuated total reflection (“ATR”) sensor (3) determines a substance dissolved in a measurement medium (2). A method is taught to verify a calibration and/or to perform an in-line calibration of the sensor. The ATR sensor includes a housing (7) and an ATR body (4). A light source (12) and a detector (13) are arranged in the housing. The ATR body has at least one measurement surface (5) and, arranged parallel to the latter, a calibration surface (10). The measurement surface (5) can be brought into contact with the measurement medium. A calibration chamber (9), arranged in the housing, is delimited on at least one side by the calibration surface.