Cluster ion formation in thermospray ionization of ammonium salts with water as solvent is investigated and used to probe the state of the gas mixture at the end of the capillary vaporizer and effects of the gas expansion into the jet chamber. Experiments with ammonium nitrate reveal the formation of cluster ions by desolvation in the capillary and point to a nearly dry state of the gas mixture at the end of the capillary. A nearly dry state of the gas is also indicated by the particular conditions at which doubly charged cluster ions of ammonium chloride could be detected. For ammonium chloride the growth of cluster ions by recondensation of volatilization products in the jet chamber is observed. This effect reveals that the gas temperature remains significantly below the jet chamber temperature up to the sampling orifice even at higher jet chamber pressures.