Interfacial properties of glass fibers coated by admicellar polymerization with a styrene-isoprene copolymer thin film have been investigated. Advancing and receding contact angles on single glass fibers as small as 6.2 mum in diameter were measured at slow meniscus velocities (2.0 mum/s) by using the Wilhelmy method with probe liquids of various hydrophobic/hydrophilic characteristics. Distinctly different contact angles were obtained from the dynamic wetting of fibers with different surface treatments. In water, the average advancing contact angle for admicellar-modified fibers was high (theta (adv) approximate to 65-75 degrees), suggesting an increased hydrophobicity as compared to that of the fully wettable desized fibers. Furthermore, it was found that both fibers with silane coupling agents and admicellar-coated fibers are chemically heterogeneous on their surfaces. Contact line pinning behavior induced by regions of different surface energy, as well as high hysteresis values, indicates that the ultrathin styrene-isoprene film is not uniform throughout the. glass on a macroscopic length scale. However, when assessed with an epoxy resin (e.g. EPON 815C), the admicellar polymerization treatment produced fibers with statistically identical wetting characteristics to those of fibers containing a proprietary sizing. Good adhesional wetting of the elastomer-modified fibers by an epoxy resin may be indicative of an adequate interaction between fiber and matrix in large-scale composite fabrication.