Ab initio molecular orbital calculations were performed to compare interdimer reaction pathways with intradimer pathways of the reactions of a H-2 molecule on C, Si, and Ge(001) surfaces. It was found that the interdimer desorptions of a H-2 molecule from H-terminated C, Si, and Ge(001) surfaces do not produce hyperthermal energy. Also, in both adsorption on the. clean surfaces and desorption from the H-terminated surfaces, the interdimer reaction pathways on C, Si, and Ge(001) surfaces have a smaller reaction barrier than the intradimer reaction pathways.