The present invention is directed to the surface treatment of silicone hydrogel contact lenses. In particular, the present invention is directed to a method of modifying the surface of a contact lens to increase its hydrophilicity by coating the lens with a carbon-containing layer made from a diolefinic compound having 4 to 8 carbon atoms. In one embodiment, an optically clear, hydrophilic coating is provided upon the surface of a silicone hydrogel lens by sequentially subjecting the surface of the lens to: a plasma oxidation reaction, followed by a plasma polymerization reaction in the presence of a diolefin in the absence of air. Finally, the resulting carbon layer is rendered hydrophilic by a further plasma oxidation reaction or by the attachment of a hydrophilic polymer chain.