Electrokinetics is a convenient tool for characterizing surface properties of solids, but also for studying adsorption processes on such surfaces. One topic of our interest was the study of the sizing process of glass fibres. With the help of streaming potential measurements using the electrokinetic analyser (EKA by Anton Paar, Graz, A), we found an interesting change of surface properties of glass fibres depending on the size composition. In case of adsorption of pure -aminopropyltriethoxisilane (AMEO) onto glass fibre surface, the resulting surface has strong basic properties with an isoelectric point () of about pH(IEP)=10. In the case of modification with a mixture of AMEO and one cationic surfactant (in our case oleyltrimethylammoniumchloride/Arquad S50), the IEP changes to pH(IEP)=6.3. The shape of the function zeta-potential vs. pH corresponds to a curve of surfaces either without any functional groups or with amphoteric properties. For us, this result was astonishing. Due to the cationic functional groups of the AMEO and the surfactant, a highly polar cationic (positively charged) surface was expected. This article discusses possible reasons for the observed behaviour.