Water and oxygen permeability measurements on t w o polymers, poly(ethy1ene terephthalate) and polypropylene, are presented as a function o f diamond-like carbon coating thickness. Results show t h a t reliable and reproducible coatings can be achieved on poly(ethy1ene terephthalate) such that levels o f permeability are about 1 cc/m2/day f o r oxygen and 1.5g/m2/ day f o r water vapour, comparable t o the levels f o r silicon oxides and aluminium coatings used in the packaging industry. The advantages conferred by diamond-like carbon over aluminium is primarily that o f retaining optical transparency in t h e thickness o f films used in this w o r k (20nm). The advantages o f diamond-like carbon over silicon oxides is related t o its intrinsic flexibility. Other advantages over other barrier films (e.g. polyvinylidine chloride) and coating technologies is t h e ability t o recycle the used product. The permeability o f diamond-like carbon-coated polypropylene t o oxygen is in t h e range of 200cc/m2/day, again comparable t o results obtained with t h e other coatings. The optimum film thickness f o r poly(ethy1ene terephthalate) t o minimize permeability was 20 nm. Atomic force microscopy revealed agglomerated structures (possibly graphitic) with t h e underlying substrate appearing smoother than t h e starting material. In comparison, polypropylene exhibited increased surface roughness under t h e same coating conditions.