The diffusion of carbon monoxide molecules on the (110) surface of copper was investigated in the temperature range between 42 and 53 kelvin. The activation energy for thermal motion was determined directly by imaging individual molecular displacements with a scanning tunneling microscope. An attractive interaction between carbon monoxide molecules gave rise to the formation of dimers and longer chains. Carbon monoxide chains diffused substantially faster than isolated molecules although the chains moved by a sequence of single-molecule jumps. A higher preexponential factor in the Arrhenius law was found to be responsible for the observed efficiency of chain hopping.