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Coadsorption of methyl-substituted cyclohexanes and CO has been investigated by temperature-programmed desorption (TPD). Coadsorbed CO promotes decomposition of cyclohexanes whose TPD spectra show two monolayer desorption peaks. The lower-temperature peak is due to the desorption from the terrace and the higher one comes from the cyclohexanes which interact with the step. By using effects of coadsorbed CO, we have determined decomposition fraction (ratio of decomposed molecules to adsorbed ones) of cyclohexanes with changing coverage of cyclohexanes. Decomposition fraction increases sharply when a tertiary C-H bond is introduced into a cyclohexane ring by methyl substitution, which indicates that introduction of tertiary C-H bond increases decomposition probability of cyclohexanes. This is consistent with the fact that dissociation energy of tertiary C-H bond is smaller than those of primary and secondary C-H bonds.