The dependence of the magnitude of the interfacial parameters for a glass-fibre-reinforced polypropylene on the thickness of the applied silane layer on the glass-fibre surface was investigated. The interfacial parameters studied included the interfacial-shear strength, the interfacial coefficient of friction, the interfacial-frictional stress and the shrinkage pressure. These parameters were evaluated from pull-out data using a recent model. The results indicate that the maximum interfacial-shear strength is obtained at a critical thickness of the silane layer on the treated fibre. Both the interfacial-frictional stress and the interfacial coefficient of friction decreased with increased thickness of the silane coat.