We use a novel low‐temperature scanning tunneling microscope to study the motion of individual CO molecules in molecule cascades on a copper (111) surface. The microscope has a base temperature of 1K in continuous circulation of3He and 0.5K in a single shot mode with a hold time of about 10 hours. The initial liquefaction of3He is achieved by utilizing Joule‐Thomson expansion of3He gas which eliminates the need for a pumped4He reservoir. Samples can be prepared and transferred into the microscope in ultra high vacuum. We found a strong dependence of the hopping rate on the carbon isotope, which leads us to the conclusion that CO molecules tunnel from initial to final state at temperatures below 6K. At higher temperatures a thermally activated process contributes to the hopping rate and several models for this process are discussed. © 2003 American Institute of Physics