Several methods for characterizing the degradation of silk fabrics that have been subjected to natural and accelerated aging by exposure to heat and light are described. Change in color, strength degradation, and increases in amino- and ammonia-nitrogen contents were measured. Among the more notable findings were that the fabrics discolor on the order of 10 times faster when degraded thermally than when degraded by light (at equal levels of degradation as measured by strength loss) and that strength loss is proportional to the increase in ammonia and amino group contents for fabrics degraded by heat or light. The amino group and ammonia contents of 16th-, 18th-, and 19th-century silks range from values typical for artificially aged contemporary silks to three times greater for amino nitrogen and four times greater for ammonia. All of the tin-weighted historical fabrics (except three dyed with logwood) have higher amino group contents than unweighted fabrics. The ammonia contents do not vary much, however.