Abstract: A method and a spectrometer for measuring the absorbance difference spectra of radical reactions using a modulated perturbation of the concentration of reactants in, for example, radical complexes induced by radiation, whereby the yield of substances and/or molecular states produced by recombination and by escaping radicals is influenced. A modulated magnetic field is exerted on the material, and the changes in absorbance of the material generated as a result thereof are phase-sensitively detected for a wide frequency range.The spectrometer is provided with means for radiation of the material to be investigated, means for receiving the same material, cooling and/or heating means, detection means for the detection of the resulting absorbance spectrum of the said material, and further with a device for exerting a modulated magnetic field on the material. The detection means have a phasesensitive detector which converts the change in absorbance generated into a dc absorbance.