Foam compositions with improved physical characteristics are from urethane prepolymers in the presence of various modifying agents. Blowing agents employed in the presence of surfactants make it possible to obtain either open or closed cell structures, and the degree of foam stiffness may be desirably controlled with polyols, as well as with polymeric polymethylene polyphenylisocyanates. Suitable modifying agents include asphalts, portland cement, epichlorohydrin, other epoxidized compounds, additional polymers, and various other materials.