The combustion processes of four fuels with gaseous O at two flowrates were investigated. The mass burning rates were determined for sheet stock and cast plexiglass, 30% Al in plexiglass, polystyrene, and polyethylene. The burning behavior of polystyrene, which was different from that of the other fuels, was explained by diffusion considerations. The mass regression rate of plexiglass decreased with burning time for the two O flowrates investigated, which was consistent with a diffusion mechanism.