The effects of coadsorbed hydrogen (or deuterium) on the dehydrogenation of cyclohexane on Pt(lll) has been studied using laser induced thermal desorption-Fourier transform mass spectrometry (LITD-FTMS), Auger electron spectroscopy (AES), and thermal desorption spectroscopy (TDS). With 0.25 of saturation monolayer coverage of cyclohexane on the crystal surface, the onset for the competing processes of desorption and dehydrogenation occurs at approximately 205 K. Our experiments show that coadsorbed hydrogen causes a shift to lower temperature of the onset of both cyclohexane dehydrogenation and molecular desorption. In the presence of saturation amounts of coadsorbed hydrogen, the onset for the two precesses occurs at approximately 165 K. In the presence of coadsorbed hydrogen or deuterium, LITD-FTMS experiments show that adsorbed cyclohexyl (C6H11) is produced in the first step of the dehydrogenation of cylclohexane. These experiments provide the first observation of cyclohexyl as a product of the thermal dehydrogenation of cyclohexane on Pt. The cyclohexyl species is stable on the Pt(111) surface at temperatures up to 205 K. In addition, H coadsorption thermal desorption spectra of cyclohexane show two desorption maxima, at 195 and 235 K. In the presence of coadsorbed H the total amount of C6H12 that desorbs is a factor of 2 more than in the noncoadsorption case. Deuterium coadsorption experiments indicate that the higher temperature desorption process is the result of the rehydrogenation reaction: C6H11 + H (D) --> C6H12 (C6H11D) In the case of deuterium coadsorption with C6H12, both C6H12 and C6H11D are observed as rehydrogenation products, the C6H12 resulting from surface hydrogen generated in the initial dehydrogenation process. In this case, both rehydrogenation products (C6H12 and C6H11D) are observed at 255 K. This observation is discussed in terms of a kinetic isotope effect for the rehydrogenation of C6H11.