The dehydrogenation and thermal decomposition mechanisms of cyclohexene and 1,3-cyclohexadiene on clean Mo(110) and carbide-modified (4 x 4)-C/Mo(110) surfaces have been studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). On the clean Mo(110) surface, partial dehydrogenation of a fraction of the cyclohexene molecules occurs at temperatures as low as 80 K. When the surface is heated to 150 K, the HREEL spectra obtained are characteristic of a C6H9 intermediate, as seen by a comparison with HREEL spectra reported for C6H9 On Pt(111).(1,2) At higher temperatures, competing C-C and C-H bond cleavage reactions lead to the formation of surface carbon and the evolution of hydrogen. In contrast, on the carbide-modified surface, the primary reaction pathway for cyclohexene is selective dehydrogenation to form benzene and hydrogen. In the case of 1,3-cyclohexadiene, the HREEL results suggest that dehydrogenation to form benzene occurs at 80 K on the clean Mo(110) surface, based on a comparison with the HREEL spectrum for benzene directly dosed onto Mo(110) at 80 K. However, upon heating, most of the benzene decomposes to form surface carbon and hydrogen, as shown by TPD studies. On the carbide-modified surface, the primary reaction pathway for 1,3-cyclohexadiene is selective dehydrogenation to form benzene, which desorbs at 313 K. Furthermore, the HREEL results also indicate that a competing reaction pathway occurs to form a surface intermediate which most likely has an tilted aromatic c-C-6 ring, such as a surface phenyl species.