Abstract We studied the thermal stability of small palladium clusters on a TiO 2(110) surface. Upon heating the number of particles decreases and the average size of the particles increases. This corresponds to a decrease of palladium surface area of this model catalyst. The decrease in particle density cannot be used to distinguish between different kinds of mass transport over the surface, but on the basis of the resulting size distribution we conclude that the coalescence of diffusing particles is the main mechanism. This is supported by a direct observation of mobile clusters. During growth the palladium clusters change their shape.