A steam expanded alkenyl aromatic polymer composition (10) is provided. The polymer is mixed with a volatile blowing agent containing, as a primary blowing agent, dichlorodifluoromethane and as a secondary blowing agent, a halogenated hydrocarbon, a hydrocarbon, or an aliphatic alcohol containing from 1 to 4 carbon atoms. The secondary blowing agent has a normal atmospheric boiling point of from 0o to 100oC. The admixture of polymer and blowing agent may be expanded by steam immediately after extrusion foaming or may be expanded after aging of the composition which has been impregnated with the volatile blowing agent.