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Abstract: Recently, it was found out that a site-specific chemical bond scission is clearly observed at the O–CH3 bonding in a methyl ester functional group of a methyl ester-terminated self-assembled monolayer (SAM) at resonant core excitations of σ *(O–CH3)←C1s(O–CH3) and O1s(O–CH3). In this work, we found that the fragmentation of desorbing CH3 + ion from a methyl ester group has a strong dependence on the atomic position of the primary core excitation. σ *(O–CH3)←C1s(O–CH3) excitation promotes the further fragmentation of CH3 + ion and produces remarkable fragment CH n + () ions, whereas σ *(O–CH3)←O1s(O–CH3) excitation suppresses such a fragmentation. These findings are important not only to elucidate the mechanism of the site-specific chemical bond scission but also to apply this technique to the future controlling of chemical reactions. [Copyright &y& Elsevier]