Abstract: In order to investigate the reactants involved in a reaction between a gas and a solid by means of X-ray absorption spectroscopy an apparatus is provided which comprises, within a measuring chamber (2), a sample holder (23) for holding a sample (24) of the solid, means for forming an atmosphere of the gas in the measuring chamber under selectable pressure, a window (20) for the entry of X-rays for the irradiation of the sample (24) and a collector arrangement (21, 22) arranged spaced from the sample holder (23), in order to collect in the measuring chamber (2) electrons released by X-ray absorption. The collector arrangement comprises two collector electrodes (21 and 22) which are arranged mutually spaced from one another in sequence lengthwise along the X-ray path between the window (20) and the sample holder (23).