A method for the selective determination of the scattering index .mu..sub.s of a scattering biological matrix, in particular for the purpose of non-invasive determination of the concentration of glucose in the skin, by means of detection measurements, in each of which light in the form of primary light (9) is irradiated into the biological matrix (5) and an intensity measurement value of secondary light (12) exiting at a detection site (33-40) that is located at different measuring distances (.rho.) from the respective light irradiation site (10) during the detection measurements is measured. In order to improve the quality and selectivity of the determination of .mu..sub.s, the primary light is irradiated obliquely at an angle between 5.degree. and 85.degree. using a contacting light-guiding element. In at least two detection measurements, the measuring distance (.rho.) between the respective light irradiation site (10) and the respective detection site (33-40) corresponds to no more than five times the mean free path length of the light propagating in the biological matrix.