The concentration of the solution within aerosol droplets exiting a jet nebulizer is determined by applying a control volume analysis to the air, water, and solute transport through the nebulizer. Measurements and calculations are made for the DeVilbiss Pulmo-Neb(R) disposable nebulizer delivering two unit dose nebules of Ventolin(R) (2.5 ml, 1 mgml(-1) salbutamol sulphate, 0.9% saline), and it is shown that the droplet solution concentration is closely approximated by the concentration of the solution remaining in the nebulizer＇s reservoir. By increasing the time the aerosol droplets have before contacting the ambient environment, it is shown that the droplets are in equilibrium as they exit the nebulizer, and therefore the concentration of the solution in the droplets is independent of droplet size.