The development of accelerated test methods to characterize long-term environmental effects on fiber-reinforced plastics (FRPs) requires the use of physicochemical methods, as well as macromechanical measurements, in order to investigate the degradation processes and predict their course over long periods of time. Thermochemical and mechanical measurements were performed on a large number of FRPs exposed to neutral, basic, and acidic media between 23 and 80°C over periods of 7 to 224 days. The resin matrices used in the present study included vinylester, polyester, and epoxy, and the fiber materials were silicate glass, aramid, and carbon. TGA was used to study the effects of aqueous media on FRPs. In particular, the relative weight loss upon heating the previously exposed material from 150 to 300°C was found to be indicative of the extent of matrix depolymerization. Indications were obtained for correlation between this weight loss and the extent of degradation of various measures of mechanical strength. The measured weight change of the tested materials during exposure was found to reflect the extent of water absorption and could be related to the extent of the weight loss between 150 and 300°C. In basic environments, weight loss, rather than gain, took place as a result of fiber dissolution. Among various FRPs, those with aramid fibers were found to be especially prone to degradation due to extensive water uptake. Basic environments caused a large decrease in strength due to fiber dissolution. Acetic acid buffers caused significant matrix depolymerization. Raising the temperature to 80°C appeared to be the most effective method of enhancing the degradation effects, as reflected in both the thermochemical and mechanical data. Furthermore, the trends observed at 80°C (for example, with respect to solvent effects) were identical with the trends observed at lower, service-like temperatures. Thus, elevating the temperature is a promising acceleration factor. In agreement with the enhanced matrix depolymerization observed in the TGA measurements, higher temperatures also caused a decrease in Tg, as measured by DSC.