X-Ray photoelectron spectroscopy can be applied to the study of the bonding of highly functionalised silicones on to E-glass fibres. The adsorption of six types of silicones (hydrido, epoxy, unsaturated imino, methacryl, 'terminal' amino and 'pendant' amino) from dilute toluene solutions were compared with that of a standard silane coupling agent. The calcium concentration was found to be an effective determinant of surface coverage by the siloxanes, as well as enabling the thicknesses of the adsorbed layers to be determined.