The adsorption of alkylphenylpoly(oxyethylene)alcohol on E-glass fibres was characterized by X-ray photoelectron spectroscopy, by the Wilhelmy plate method and by streaming-potential measurements. In this study, the glass fibres were conditioned in solutions of different pH. The acid resistance of the E-glass fibres was studied by treating the material with H2SO4. During the acid treatment the surface composition, namely the Si/Al-ratio, was changed, presumably because the aluminium ions were leached out of the glass structure. The alkaline resistance was studied by treating the material with NaOH. The Si/Al-ratio was unaffected by these moderate alkaline solutions. The effect of the surface conditioning in solutions of different pH seemed to be greatest for the acidic solutions giving rise to an increased adsorption of the surfactant on the glass-fibre surface.