The surface properties of three glass fibers made from silicate waste and of various commercial glass fibers have been characterized by zeta (zeta)-potential and contact angle measurements. zeta-Potential measurements indicate that the formation of the electrochemical double layer is clearly affected by the type and amount of network former and network modifier oxides in the glass structure. A high negative zeta-potential is measured for fibers containing large amounts of network former oxides. The amount and type of network modifier cations leaching from the glass and adsorbing in the electrochemical double layer causes the negative zeta-potential to decrease. pH-depending zeta-potentials show that the Bronsted acidic surface character predominates. The ratio of network former to network modifier oxides affect the course of the zeta = f(pH) function and, therefore, the acidity of the fiber surfaces. Contact angles of glass fibers against water and diiodomethane have been measured in order to estimate the fiber surface tensions. All investigated glass fibers are rather hydrophobic. The surface tension of the fibers is similar to polymer. The results of the present study are relevant for the use of such glass fibers as reinforcement for polymer matrix composites. (C) 2004 Kluwer Academic Publishers.