While reactions on the surface of bulk metal proceed in general under well-defined and experimentally easily measurable temperature and other conditions, this is usually not the case for reactions of metal atoms or clusters and their ions in high vacuum. A collision complex of the metal cluster with a polyatomic molecule (e.g., benzene) may be transiently stabilized by redistribution of the interaction energy, IVR. This energy redistribution results in heating of the collision complex to internal temperatures, which can be estimated to be often very high, but the data for its exact determination are usually not available. Semiquantitatively, the heat capacity of the complex will increase with the cluster size, and the temperature rise upon complex formation will correspondingly decrease. Such size-dependent heating effects are readily observable in FT-ICR studies of anionic and cationic cluster reactions, and when interpreting the data or trying to draw from it conclusions for bulk condensed phase reactions, they have to be taken into consideration. Conversely, the effect of such temperature rise in mass spectrometric studies can be decreased or eliminated, and the reactions of truly cold clusters can be investigated by "soft landing" the reactive molecule on a solvated cluster and exchanging it for inert ligands, e.g., argon atoms.