The preparation of a collection of supported metallic particles that are truly monodisperse, i.e. they all have exactly the same size, has long considered to be virtually impossible. As a consequence it has been difficult to recognize size effects for small supported clusters, as size distributions have been broad. For clusters in the gas phase, however, strong size-dependent chemical properties have been discovered during the last two decades and the obtained results lead to new concepts for understanding their chemical properties. Connections of these observations with real catalysis has often been stressed and suggestions for tuning efficiency and selectivity of a certain catalytic process by simply changing cluster size were made already in early days. Efficient and selective conversion is indeed important in catalysis, as most catalytic surfaces assist a variety of reactions. It is therefore of interest to study the factors affecting the size-dependent selective and efficient behavior of catalytic systems. We succeeded to prepare model catalysts consisting of a collection of metal particles of a single size. In these experiments metal atoms and small metal clusters are formed in the gas phase, size-selected and then deposited on thin MgO and TiO2 films grown on metal surfaces. Various chemical reactions on the obtained cluster-assembled materials are then investigated under UHV conditions by means of thermal desorption and infrared spectroscopies. Oxidation and polymerisation reactions are strongly dependent on cluster size and on the cluster-support interaction, and not only the number of product molecules per cluster is changed, but also the branching ratio of certain reactions. In many cases the measured reactivities are different from the ones obtained for the corresponding bulk systems. By combining the obtained experimental results with ab-initio calculations, a picture of the size-dependent behavior of cluster-assembled materials is now emerging.