The adsorption of acetylene on the Ge(001) surface is investigated by first-principles density-functional calculations within the generalized gradient approximation. We find that the di-sigma structure is energetically favored over the end-bridge, r-bridge, and p-bridge structures, while the paired end-bridge structure is the most stable. Especially, our calculated adsorption energy for the p-bridge structure is -0.03 eV, indicating that this structure is not stable on Ge(001). This result does not support the conclusion drawn from a recent scanning tunneling spectroscopy (STM) and temperature programmed desorption experiment where the most populated adsorption configuration was assigned to the p-bridge structure. Our calculated energetics and STM simulation of various adsorption configurations are consistent with the two observed configurations being the di-sigma and paired end-bridge structures. (C) 2003 American Institute of Physics.