This study is of toughening some vinyl ester resins with low-molecular-weight butadiene-based polymers. Liquid polybutadiene and copolymers with acrylonitrile are immiscible with the commonly used vinyl ester resin, Derakane 411-45. Despite this incompatibility, some toughening is seen in blends with this vinyl ester resin. Modification of hydroxy-terminated polybutadiene through reaction with a diisocyanate and various alcohols renders the polymer more compatible but not miscible with the resin. With increasing length of the end-groups, the ability to provide a tougher cured resin improves such that the energy to propagate a slow crack increases more than 10 times. Addition of 5 wt% of a butadiene polymer resulted in a disperse-phase volume fraction greater than 10%. Blends with Derakane 411-45 showed greater toughness than that of the toughened resin Derakane 8084, which was also improved by addition of butadiene copolymers.