A section of a carbon-fiber-reinforced composite horizontal stabilizer skin from a 737-200 aircraft that had been in service for 20 years was analyzed with Fourier transform infrared spectroscopy and dynamic mechanical thermal analysis in an effort to determine the molecular changes that occurred in the epoxy matrix resin over the service life. Comparisons were made with a similar matrix resin system that had been aged under various accelerating conditions. As expected, the results showed that the molecular changes were slight and occurred only on the unpainted (internal) surface areas of the composite. The changes in the commercial materials during the in-service aging were most similar to those in a composite that had been artificially aged at 120 degrees C for 3000 h, but they included two chemical changes not seen previously. There was an increase in the number of aliphatic hydrocarbon molecules (a fuel chemical) as well as a decrease in the number of molecules containing -SO2- units. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 2981-2985, 2010