Abstract: The invention relates to a method and a device for identifying active substances in order to determine the formation of complexes between reactants. According to the inventive method, at least two reactants are mixed and made to react to form a complex. The infrared spectrum of the individual reactants which have not yet been reacted in the mixture is measured during a first time interval and at least a second infrared measurement is conducted to measure the complex formed by the reactants during a second interval. The difference between the two spectra measured during different moments is then determined. The reactants whose differential spectrum exhibits a band structure are selected as active substances.