The effect of nebulizer solution temperature and dilution air humidity on the size and solute concentration of aqueous aerosol droplets were studied. Four combinations of jet-nebulizers with air compressors or oxygen sources and one ultrasonic nebulizer were tested. The temperature to which the nebulizer solution of each system fell during generation was measured. The nebulizers were then kept at set temperatures, generated aerosols collected and either droplet size or solute concentration measured. The droplet solute concentration was found to increase. The droplet size decreased along with the droplet solute concentration increase. The ultrasonic nebulizer also was tested: its high output made the concentration of the solution in the droplets much more stable. However, the proportion of droplets depositing in the tubing and valves changed markedly with aerosol flow rate. The potential for large changes in droplet solute concentration, droplet size and output during nebulization should be considered in therapeutic and diagnostic applications of nebulized aerosols.