The effects of propellant type, cosolvent content, and air humidity on the morphology and solid phase of the particles produced from solution pressurized metered dose inhalers containing the corticosteroid beclomethasone dipropionate were investigated. The active ingredient was dissolved in the HFA propellants 134a and 227ea with varying levels of the cosolvent ethanol and filled into pressurized metered dose inhalers. Inhalers were actuated into an evaporation chamber under controlled temperature and humidity conditions and sampled using a single nozzle, single stage inertial impactor. Particle morphology was assessed qualitatively using field emission scanning electron microscopy and focused ion beam-helium ion microscopy. Drug solid phase was assessed using Raman microscopy. The relative humidity of the air during inhaler actuation was found to have a strong effect on the particle morphology, with solid spheroidal particles produced in dry air and highly porous particles produced at higher humidity levels. Air humidification was found to have no effect on the solid phase of the drug particles, which was predominantly amorphous for all tested formulations. A critical level of air relative humidity was required to generate porous particles for each tested formulation. This critical relative humidity was found to depend on the amount of ethanol used in the inhaler, but not on the type of propellant utilized. The results indicate that under the right circumstances water vapor saturation followed by nucleated water condensation or ice deposition occurs during particle formation from evaporating propellant-cosolvent-BDP droplets. This finding reveals the importance of condensed water or ice as a templating agent for porosity when particle formation occurs at saturated conditions, with possible implications on the pharmacokinetics of solution pMDIs and potential applications in particle engineering for drug delivery.