We investigated the adsorption and decomposition of thiophene (C(4)H(4)S) on Ge(100) using high-resolution photoemission spectroscopy. We found that the Ge 3d and C 1s core-level spectra revealed three adsorption geometries, which we assigned to a weakly bound state (i. e., a Ge-S dative bonding state), a [4+2] cycloaddition bonding state, and a decomposed bonding state (a desulfurization reaction product) as functions of the molecular coverage and the annealing temperature. In this study, we systematically elucidated the changes occurring in the bonding states of thiophene species adsorbed on a Ge(100) substrate.