The adsorption and decomposition mechanisms for 1-propanethiol on a Ga-rich GaP(001) (2 X 4) surface are investigated at an atomic level using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. Using a combination of experimental and theoretical tools, we probe the detailed structures and energetics of a series of reaction intermediates in the thermal decomposition pathway from 130 to 773 K. At 130 K, the propanethiolate adsorbates are observed at the edge gallium sites, with the thiolate-Ga bonding configuration maintained up to 473 K. Further decomposition produces two new surface features, Ga-S-Ga and P-propyl species at 573 K. Finally, S-induced (1 x 1) and (2 X 1) reconstructions are observed at 673-773 K, which are reportedly associated with arrays of surface Ga-S-Ga bonds and subsurface diffusion of S. To understand the observed site selectivity on the hydrogen dissociation of the thiol molecule at 130 K, the two most likely dissociation pathways (Ga-P vs Ga-Ga dimer sites) are investigated using DFT Gibbs energy calculations. While the theory predicts the kinetic advantage for the dissociation reaction occurring on the Ga-P dimer (Lewis acid-base combination), we only observed dissociation products on the Ga-Ga dimer (Lewis acid). The DFT calculations clarify that the reversible thiolate diffusion along the Ga dimer row prevents recombinative desorption, which is probable on the Ga P dimer. Together with experimental and theoretical results, we suggest a thermal decomposition mechanism for the thiol molecule with atomic-level structural details.