Hyperbranched resins have been evaluated for use as barrier coatings. Different types of modified hydroxy-functional hyperbranched polyester resins were prepared and partially end-capped with acrylate or methacrylate units. Crystalline end-groups (poly(epsilon -caprolactone) were grafted onto some resins. The coatings were applied on poly(ethylene terephthalate) and polypropylene substrates and crosslinked by UV light and their barrier against oxygen and water vapour determined. The results show that hyperbranched coatings give a pronounced improvement in oxygen barrier on both substrates. Expressed in terms of oxygen permeability coefficient, the improvement was a factor of up to 60 times compared to polypropylene and up to 2.2 times compared to poly(ethylene terephthalate). As for other hydroxy-functional barrier polymers, the amount of hydroxyl groups in the hyperbranched resins was found to play a major role concerning the oxygen barrier, controlling both performance and water sensitivity. It was seen that humidity led to a decrease in barrier at the highest hydroxyl group concentration, whereas the barrier performance of coatings with lower concentration of hydroxyl groups instead improved under humid conditions. It was also found that crystalline end-groups did not improve the oxygen barrier and that none of the synthesised resins gave any improvement in water vapour barrier.