The adsorption of silane coupling agents onto glass fiber surfaces has been investigated. The type of adsorption was elucidated using electron spectroscopy for chemical analysis (ESCA or XPS). The surface charging was recorded using streaming potential analysis. The silane bond strength was tested by boiling the silanized fibers in water for 2 h. Thereafter the conductivity of the water was measured in order to estimate the capability of the silane surface film to prevent ion dissolution from the glass. ESCA provided information on the amount adsorbed and indicated that substantial rearrangement in the surface film structure occurred as a function of the silane concentration. The aminosilane produced a strong positive charge on the glass fibers, while the nonionic silanes were only partly condensed, giving rise to a substantial enhancement of the negative charge. The conductivity measurements indicated that the silane films were present as a loose patch-like silane network on the surface of the E-glass fibers. This conclusion is in accordance with the results obtained with all the techniques used.