We find that TEP is adsorbed molecularly and TBP is partially dissociated at RT. Some of the TEP molecules on the surface are desorbed and the others are decomposed into C2H4 and H2, evolving from the surface. The desorption peak of the TEP molecule is increased in intensity under hydrogen ambience. For TBP, the TPD peaks for C4H9 radical and C4H8 are found. The peaks of C4H9 radical and C4H8 are increased and decreased in intensity, respectively, under hydrogen ambience. The decomposition mechanisms of TEP and TBP on the GaP(001) surface are discussed.