The adsorption and thermal decomposition of 1,3-butadiene has been studied on Ru(001). Starting with a condensed multilayer at 80 K, the decomposition of I,3-butadiene was monitored as a function of temperature using temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Annealing the multilayer-covered surface to 800 K results in the decomposition of some of the adsorbed 1,3-butadiene, leaving a surface carbon coverage corresponding to 0.67+/-0.03 carbon adatoms per surface ruthenium atom, as determined by titration with oxygen. As the butadiene decomposes between 230 and 700 K, hydrogen desorbs in an amount corresponding to 1.01+/-0.05 adatoms per surface ruthenium atom. A HREEL spectrum taken after annealing to 260 K for 10 s provides spectroscopic evidence for the presence of a (CH), metallacycle decomposition product. This assignment is consistent with and supported by the determination of a surface stoichiometry of CH1.04+/-0.11 using TPD spectra. The formation of the catalytically important (CH)(4) metallacycle has been predicted theoretically, but this work represents the first experimental evidence for the formation of this intermediate from the decomposition of 1,3-butadiene.