The aim of this work is to study the influence of moisture on the surface chemistry and texture of C and E glass fibers. Thermal analysis (in the conditions of controlled rate transformation analysis) coupled with mass spectroscopy, surface area determination (by krypton adsorption at 77 K), adsorption of water by gravimetry, and AFM were used to characterize both novel and aged fibers (a few days at 50 degrees C in 95% humidity). Whereas aging has little effect on E glass fibers which can be interpreted by the appearance of both a small roughness and hydrophilic superficial sites, the changes observed on the C glass fibers are drastic. The surface area is increased by at least 10 times, micropores appear (as shown by gas adsorption), and clusters (evidenced by AFM) are formed on the surface. The migration of sodium and calcium from the bulk toward the surface (where they are stabilized under the form of carbonates in agreement with thermal analysis results) could explain the formation of both micropores and clusters.