Room temperature ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) is used to characterize the motion of individual organic molecules on the Si(100)-2 X 1 surface. In particular, 4-methoxystyrene molecules are observed to translate laterally on the surface during UHV STM imaging. Switching between the two most favored conformations occurs on the time scale of 0.1-1 s. On the other hand, styrene molecules imaged under identical conditions are not observed to undergo lateral translations, thus suggesting that the rotational freedom of the methoxy group is enabling the apparent motion of 4-methoxystyrene. To test this hypothesis, the rotational freedom of the methoxy group was eliminated by synthesizing an analog molecule (5-vinyl-2,3-dihydrobenzofuran) where the methoxy group was covalently linked back to the aromatic ring. UHV STM studies of 5-vinyl-2,3-dihydrobenzofuran confirm the expected suppression of molecular motion. Overall, this study suggests that the motion of surface-mounted adsorbates can be controlled by engineering intramolecular rotational degrees of freedom. (c) 2005 American Vacuum Society.