Metal ion catalysis of isocyanate-hydroxyl reactions is considerably enhanced by the presence of certain "activating" groups a, 6, even y to the carbon carrying the reactive hydroxyl. Few metal ions can significantly catalyze the isocyanatebutanol reaction. In general the metal ion catalysts fall into two groups: catalysts which maintain a fair portion of their activity, even in system containing hindered isocyanates, and catalysts which are very sensitive to changes in steric factors. It is believed that metal ions fulfill a twofold function: they increase the electrophilic character of the isocyanate group by coordinating either at the oxygen or nitrogen, and they bring the isocyanate and alcohol in closer proximity. The "activating" groups are believed to have two major opposing effects: to increase the overall stability of the ternary complex while allowing the hydroxyl group to retain most of its nucleophilicity and to produce in some cases urethanes which preferentially chelate the metal ion and kill the catalpis. All metal ions exhibit greater catalytic activity in systems containing 1,Z-than l,4glycols; furthermore the sensitivity to steric hindrance increases from 1,s to 1,4glycols, indicating t.hat only chelation can sufficiently increase the stability of the ternary complex to produce the maximum catalytic effect.