Novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) by the use of temperature programmed desorption (TPD), ultraviolet photoelectron spectroscopy (UPS), and spot-profile-analysis low energy electron diffraction (SPA-LEED). The desorption energy of deuterated cyclohexane (C₆D₁₂) is lower than that of C₆H₁₂. In addition, the work function change by adsorbed C₆D₁₂ is smaller than that by adsorbed C₆H₁₂, indicating that C₆D₁₂ molecules are slightly more distant from the surface than C₆H₁₂ molecules (vertical geometric isotope effect). These isotope effects originate from a shallower adsorption potential of C₆D₁₂ than that of C₆H₁₂. The lateral geometric isotope effect was also observed in the two-dimensional cyclohexane superstructures as a result of the different repulsive interaction between interfacial dipoles. The observed isotope effects should be ascribed to the quantum nature of hydrogen involved in the C−H⋅⋅⋅metal interaction.