Temperature-programmed x-ray photoelectron spectroscopy was used to study the thermal chemistry of acetylene (C2H2) and ethylene (C2H4) on Ni(100) in the temperature range 90-530 K. The use of a third generation synchrotron light source facilitated the measurement of high-resolution C 1s photoelectron spectra within a few seconds, approaching the ideal of real-time analysis. In a quantitative and quasi-continuous manner, the thermal dehydrogenation pathways are followed. For the acetylene decomposition, acetylide (CCH) and methylidyne (CH) are confirmed as intermediates. For the dehydrogenation of ethylene, a vinyl species (C2H3) is observed. Using the fingerprint capabilities of x-ray photoelectron spectroscopy, acetylene can be identified as the subsequent dehydrogenation product. Upon further heating, acetylide and methylidyne are successively formed on the surface, as in the decomposition experiment starting with acetylene adsorbed at 100 K. For both systems carbidic carbon is formed as the final dehydrogenation product, although with different transition temperatures. Species identification is based on observed vibrational fine structure data and correlation of core-level binding energies with previous literature. (C) 2003 American Institute of Physics.