This study investigates the dissolution kinetics of the poorly water-soluble active pharmaceutical ingredient fluticasone propionate. Dissolution is a key step in the delivery of drug molecules to their site of action and is often the rate-limiting step in their pharmacokinetics. The study found that the dissolution kinetics of fluticasone propionate aerosolised from a pressurised metered dose inhaler and three dry powder inhalers could not be described by a diffusion-limited model but could be described if surface transport kinetics were also incorporated. The use of this mixed kinetic control model allowed geometric parameters describing the dissolution process to be extracted which correlated well with measurements of the agglomeration state of the particles. The study suggests that more detailed kinetic models are required to understand the implications of dissolution kinetics on the rate at which poorly water-soluble active pharmaceutical ingredients become available at their site of action.