The effect of thermal exposure on grain-boundary precipitation in Al-Cu-Mg-Ag alloy was studied using quantitative transmission electron microscopy. Grain-boundary precipitate parameters, such as average size, number density and precipitate-free zone width, were measured. The effective diameter of precipitates, number of precipitates per grain-boundary area and area fraction of precipitates on the grain boundary were calculated. These data were applied to a grain-boundary fracture model to calculate grain-boundary fracture strain. The calculated fracture strains, in turn, were used to check the validity of two existing models of fracture toughness, which are based on grain-boundary nucleation of cracks and their propagation through precipitate-free zones. The fracture toughness model of Hornbogen and Graf closely agrees with the experimental results.