Cycloolefinic polymer compositions exhibiting an advantageous balance of dimensional stability, improved impact resistance and superior optical properties are provided. The cycloolefinic polymer compositions of the invention generally include at least one cycloolefinic polymer and an effective amount of an elastomeric copolymer having an aromatic vinyl content of from 14 to 39 weight percent. The cycloolefinic polymer compositions may be incorporated into a variety of products, including multilayer films.