Bis-(salicylaldehydato)dioxomolybdenum complexes show excellent catalytic activity for the ring-opening polymerization of epsilon-caprolactone. Of the tested, the bis-(5-methoxysalicylaldehydato)dioxomolybdenum complex exhibited the highest reactivity. With 0.0005 mol% catalyst, the ring-opening polymerization was complete within 3 h at 130 A degrees C. The turnover frequency of the reaction was calculated to be 23,000 and 67,000 mol h(-1) at 110 and 130 A degrees C, respectively. After an induction period, the reaction proceeds in a living manner, following first-order reaction kinetics.