The principle and experimental setup of time‐resolved photoelectron spectroscopy are outlined. In comparison to conventional molecular beam relaxation spectrometry a reaction channel is analyzed not only at the end but prior to the end. In at least three points time‐resolved photoelectron spectroscopy is superior to photoelectron spectroscopic experiments under static and direct current adsorption conditions: (1) a general increase in detection power of transient adsorbates and contaminations; (2) the possibility to discriminate by individual relaxation time between species that cannot be resolved in their binding energy; and (3) a significant simplification of the mathematical formalism in comparison to conventional molecular beam relaxation spectrometry and a more straightforward understanding of the results, in particular the chemical identification of transient species. This is illustrated with time‐resolved UV‐photoelectron spectra of intermediate adsorbed carbon in the catalytic decomposition of methanol on iron and with time‐resolved x‐ray photoelectron spectra of intermediate adsorbed oxygen in adsorption and catalysis of methanol/oxygen mixtures on iron.