Glass fibers slowly dissolve and age when exposed to water molecules. Such a phenomenon also occurs when glass fibers are inside fiber-reinforced composites protected by the matrix. This en-vironmental aging results in the deterioration of the mechanical properties of the composite. In structural applications, GFRPs are continuously exposed to water environments for decades (typically design lifetime is around 25 years or even more). During their lifetime, these materials are affected by various temperatures, pH acidity levels, mechanical loads, and the synergy of these factors. The rate of the degradation process depends on the nature of glass, sizing, fiber orientation, and environmental factors such as acidity, temperature, and mechanical stress. In this work, degradation of typical industrial grade R-glass fibers, when inside an epoxy fiber-reinforced composite, is studied experimentally and computationally. A Dissolving Cylinder Zero-Order Kinetic (DCZOK) model was applied and could describe the long-term dissolution of glass composites, considering the influence of fiber orientation (hoop vs transverse), pH (1.7, 4.0, 5.7, 7.0, and 10.0), and temperature (20, 40, 60, and 80 °C). The limitations of the DCZOK model and effects of sizing protection, accumulation of degradation products inside the composite, and water availability were discussed. Experimentally dissolution was measured using ICP-MS. Like for the fibers, for GFRPs also, the temperature showed an Arrhenius-type influence on the ki-netics, increasing the rate of dissolution exponentially with increasing temperature. Similar to fibers, GFRPs showed a hyperbolic dependence on pH. The model was able to capture all of these effects, and the limitations were addressed.