Silane coupling agents have extensively been used as adhesion promoters in the GRP ( Glass Reinforced Plastic) composites. In recent years, the application of silanes in aluminium bonding process has been extensively investigated. Silane surface treatment is a more environment-friendly alternative than other treatments, which use toxic chemicals ( hexavalent chromium compounds, strong acids, etc.), such as FPL (Forest Products Laboratory) etch, CAA (chromic acid anodisation) or PAA (phosphoric acid anodisation). The aim of this work was to optimize the gamma-methacryloxypropyltrimethoxysilane application process to obtain the best durability in toughened acrylic bonded aluminium joints. Variables such as pH, concentration of silane in aqueous solution, drying temperature and pretreatment prior to silanisation have been investigated. The durability was tested using single-lap shear specimens and wedge tests. Specimens were exposed to 95% RH at 50 degrees C, for 90 days. Silanisation parameters such as solution concentration and drying temperature showed a great influence on the durability; however pH had only a little influence. Adherend surface preparation prior to silanisation was found to be a significant factor. The best bond durability performance was provided by the P2 etch and grit blasting, which performed better than the other pretreatments studied. (C) Koninklijke Brill NV, Leiden, 2008