The stability of plasma polymerised layers has become important because of their widespread use. This study explored the hydrolysis and degradation stability of coatings of plasma polymerised maleic anhydride. Coatings made with different plasma parameters were exposed to aqueous media of different pH as a function of time. ATR-FTIR was used for structure analysis and a toluidine blue staining method allowed quantitative analysis of the hydrolysis of anhydride groups to acid groups. Coatings with constant thickness were obtained at different plasma powers and layers with varying thickness were obtained at constant plasma power by adjusting the polymerisation time. The results show that the hydrolysis resistance of the modified layer is determined by the power used in the plasma polymerisation, while changes in the chemistry of the modified layer are insignificant. (C) 2008 Elsevier B.V. All rights reserved.