Fundamental to an atomistic understanding of many surface phenomena is how atoms and small atomic clusters behave on the surface and what energies are involved in various atomic processes. Using the field ion microscope, we have measured diffusion parameters of small Pd clusters with two to nine I'd atoms on the W(I 1 0) surface. The average activation energy of the center of mass diffusion increases with the number of Pd atoms in the clusters. From the probability of observing different atomic configurations during diffusion, possible cluster diffusion mechanisms are probed. A small I'd cluster can form either a one-dimensional (I D) linear-chain or a two-dimensional island-like structure. The stability of linear chains remains until the number of atoms reaches eight. From the probability of observing different structures approaching thermal equilibrium, we discuss the importance of many body effects in the adatom-adatom interaction. (C) 2003 Elsevier Science B.V. All rights reserved.