The metallization of polyacrylonitrile (PAN), poly(styrene-co-acrylonitrile) (SAN) and polystyrene (PS) by nickel has been investigated by x-ray photoelectron spectroscopy. Even if there is no significant loss of chemical elements during the metallization process, surface degradation is systematically seen by the presence of amorphous carbon at the interfaces. Nickel oxidation is detected by the presence of metal nitride (with PAN and-SAN) and its concentration is directly proportional to the initial nitrogen concentration in the substrates. The analysis of metallized polymer surfaces exposed to air indicates that the changes induced at the surface of the polymer by the nickel condensation are irreversible and suggests the presence of a real metal nitride at the interface. The behavior of SAN under metallization is the sum of the behavior of PS and PAN; this copolymer can merely be represented as a mixture of styrene and acrylonitrile repeat units with no particular effect due to their distribution in the macromolecular chain.