Abstract: The invention relates to a method and to an apparatus for determining the adsorption of a gas at materials. It is the object of the invention to propose possibilities for the determining of the surface properties of materials in which statements can be obtained and very small sample volumes can be examined with sufficient measurement precision, with a reduced technical plant effort and with a reduced time effort. In the invention, a sample of a material is acted on by a gas or gas mixture within a chamber which is not transparent for electromagnetic radiation in the wavelength range between 150 nm and 25 ?m. The gas or at least a gas included in a gas mixture is adsorbed at the surface of the sample and in this respect the electromagnetic radiation emitted by the sample as a consequence of the adsorption is detected by at least one optical detector which is sensitive at least in a range of the wavelength range between 150 nm and 25 ?m.