The invention relates to a method and a device for the non-invasive optical in-vivo determining of the glucose concentration in flowing blood in a blood vessel inside a body, wherein the body is irradiated with ultrasonic radiation with an ultrasonic frequency to mark a blood vessel, wherein the body with the blood vessel is illuminated with light having at least one first light wavelength, wherein the intensity of the back-scattered light depends on the glucose concentration, wherein the body with the blood vessel is illuminated with light having a second light wavelength that lies in the range of a water absorption line, the position of which depends on the temperature of the blood, wherein the respective back-scattered light is detected by at least one detector, wherein, using an evaluation unit, respective signal portions modulated by a modulation frequency depending on the ultrasonic frequency are extracted from the detector signals measured at the detector, wherein an indicator value for the glucose concentration is determined from the signal portion determined at the first wavelength, wherein the indicator value is corrected by the signal portion of the second light wavelength for the compensation of the temperature dependency.
