We have investigated the deuterium isotope effect on phase separation in aqueous zipper type hydrogen-bonding polymer solutions. The phase separation temperature of poly(acrylic acid)-poly(acrylamide) in heavy water is about 16 degrees C higher than that in water. This large isotope effect in the aqueous zipper-type macromolecular system arises from the polymer-polymer interaction due to cooperative hydrogen bonds in the polymer-polymer complex.