The disclosure is a probelike apparatus (16) for spectroscopic analysis of a fluid medium (19) by attenuated reflection. Two light beams from a light source (11) impinge upon the boundary (18) between a prism (17) and the medium (19) to be analyzed and the intensities of the light beams reflected at the boundary are measured in a detector unit (22). The two light beams differ in their angle of incidence on the boundary and/or in their polarization state. Measurement is preferably carried out under total reflection.