A transiently supersaturated drug delivery system has the potential to enhance topical drug delivery via heightened thermodynamic activity. The aim of this work was to quantify the degree of saturation (DS) for transiently supersaturated formulations using three traditional and one novel in vitro assessment methods. Metered dose aerosols (MDA) were formulated containing saturated levels of beclomethasone dipropionate monohydrate (BDP) or betamethasone 17-valerate (BMV) within a pressurised canister, and included ethanol (EtOH), hydrofluoroalkane 134a propellant and poly(vinyl pyrrolidone). Attempts to determine the DS via the measurement of drug flux through synthetic membranes did not correlate and was shown to be dependent on the EtOH concentration. The inability of these methods to accurately assess the drug DS may be due to the transient nature of the formulation and the volatile solvents dehydrating the membrane. A mathematical equation that used the evaporation rate of the formulation was derived to determine the theoretical DS at various time points after MDA actuation. It was shown that the MDAs became supersaturated with a high DS, this enhanced drug release from the formulation and therefore these preparations have the potential to increase the amount of drug delivered into the skin.