A process is provided for preparing propellant compositions including a film-forming synthetic polymer that are capable of forming foamed structures containing open and/or closed cells, which may optionally contain a material which is deposited in the pores and/or walls of the structure as the structure is formed, which comprises coating the synthetic polymer in particulate form with an inert solid material insoluble in the propellant and in solutions of the synthetic resin the propellant at atmospheric temperature; and then adding the propellant and dissolving the synthetic polymer in the propellant. The process is of particular application for preparing such synthetic polymer-propellant compositions in situ in closed containers capable of withstanding an internal pressure sufficient to keep the propellant in the liquid phase at atmospheric temperature, and when the composition is withdrawn from the container to atmospheric pressure, the propellant volatilizes rapidly and a foamed structure is formed within a few seconds.