Abstract: An optical sensor element for detecting organic compounds, particularly hydrocarbon. The sensor element permits the selective detection of substances by use of units which produce spectral displacements. This is realized by employing a spectral band filter, which comprises at least one layer of variable optical thickness of .lambda./4 and .lambda./2, respectively, or a multiple thereof, and said layer exhibits hydrophobic properties and includes organic affinity groups, the selection of which is adaptable to the substances to be detected.