Room temperature polymerization of isocyanate initiated with some lanthanoid alkoxides is demonstrated. Yttrium isopropoxide is an effective initiator for the homopolymerizarion of hexyl isocyanate at room temperature and affords the polymer a high yield, in contrast to lanthanum isopropoxide (La(O1Pr)3) which produces only cyclic trimer. On the other hand, copolymerization of hexyl isocyanate and 3-phenylpropyl isocyanate initiated with La(O1Pr)3proceeds even at room temperature. La(O1Pr)3– active hydrogen compound systems also serve effectively in the polymerization of hexyl isocyanate at room temperature.