The delivery of medicinally active compounds to the respiratory tract requires that the natural aerodynamic filtration capacity of the lung is overcome, and therefore poses a major challenge to pharmaceutical formulation. Metered dose inhalers (M Dl's) formulated as suspensions of a micronised drug in a chlorofluorocarbon (CFC) blend represent a major device for the delivery of locally active drugs used in the treatment of respiratory disorders. Despite the identification and optimisation ol the major parameters which influence the performance of MDI's such as particle size, vapour pressure, surfactant concentration (Poll et al, 1969; Moren, 1978; Newman et al, 1982), their use in practise results in extensive extrapulmonary deposition with frequently <10% of the metered dose actually reaching the targeted site. Whilst poor coordination by the patient between the commencement of inspiration and actuation of the device contributes to their poor efficacy, most of the aerosol output immediately after actuation is, in fact, aerodynamically unfavourable for pulmonary deposition (Moren and Anderson, 1981 ). The design of more elaborate actuators to encourage efficient breakup and/or collection of non-respirable aerosols droplets (Byron et al, 1989), though logical, is largely restricted by the presence of micronised drug particles within the formulation. Homogeneous systems, where the drug is dissolved in the propellant blend offers potential advantages both in formulation and in actuator and valve design which, prospectively, could lead to an improved therapeutic efficacy of this dosage form. Amphiphatic molecules in apolar media may associate into reversed or inverted micellar solutions (Luisi et al, 1988) which are homogeneous and optically transparent and the ability of such systems to solubilise quantities of water has been well docu me nted (Arkin and Singleterry, 1949; Keh and Valeur, 1981 ). The result of incorporating water into such systems is the production of a water-pool and the formation of swollen micelles, which could serve as centres for the solubilisation of drugs of various physico-chemical characteristics. We have investigated the aggregation of a soya lecithin (SPC) in a model CFC, trichlorotrifluoroethane (P113), using an iodine solubilisation method and determined the potential to solubilise water within their polar interior. Spectroscopic and viscometric techniques were employed to determine aggregation number of SPC solutions and to study the effect of solubilised water (expressed as the molar ratio of water:surfactant, R) on this and the size and shape of the reversed micelles. Drug solubility studies for a model hydrophilic compound were performed over a range of values of R and in separate experiments, the influence of added water on the micellar core polarity was investigated using a fluorescence probe. Finally, pressurised packs containing a range of surfactant concentrations and solubilised water have been formulated and the aerosols produced from those units following activation through a number of various orifices characterised.