Studies of the formation and growth of large protonated water clusters in a fast flow tube reactor at temperatures equivalent to those under which NLC are known to form offer, for the first time, the possibility to experimentally investigate some of the mechanisms of NLC formation at the molecular level. Importantly, bimodal cluster distributions obtained in the laboratory experiments are similar to those found in situ and also are in accordance with predictions based on ion-induced nucleation theories. The experiments show a strong dependence of temperature and water partial pressure on the production of large protonated water clusters of sizes similar to those observed during NLC displays. A consideration of the thermodynamics of the clustering reactions and their unimolecular dissociation kinetics provides a further understanding of the conditions necessary for the observation of the protonated water clusters both in the laboratory and in situ. Investigations of chemical reactions of the protonated water clusters with a variety of neutral reactants were also made, and the findings enable an assessment to be made of the chemical environment existing during NLC formation. ¿ American Geophysical Union 1991