The rearrangement of layers of ligand-stabilized Au-55 clusters, deposited on graphite and mica substrates, was imaged during heating in ultrahigh vacuum. No thermally induced rearrangement of the clusters was observed below a certain decomposition temperature. Significant modifications of the layer structure were found on both substrates beyond that temperature. The observed critical temperature agrees fairly well with the decomposition temperature obtained from calorimetry measurements on solutions and pallets. At higher temperatures the cluster decomposition and aggregation processes differ significantly for the two substrates. This is attributed to a much stronger cluster-substrate interaction for mica in comparison to graphite. (C) 2002 Elsevier Science B.V. All rights reserved.