Abstract Data have been presented bearing on the hydrolytic stability of a wide variety of urethan elastomers (Table III). It has boon shown that polyester based elastomers are considerably more susceptible to hydrolytic attack than polyether elastomers up to 158° F. The nature of the attack and results obtained are profoundly influenced by the temperature. At temperatures of 120° F and above the attack appears to be a “simple” hydrolysis consisting of scission of main chain ester groups resulting in reversion of the rubber. In this case aging over water and in water are similar. At temperatures below 120° F the attack cannot be rationally treated without considering the part played by fungi. In this case exposure over water yields drastically different results from immersion— the former being more severe by several orders of magnitude. Typically the attack over water involves a short term (two to three weeks) cracking of stressed or unstressed specimens and a gradual softening which takes place over a period of many months. The cracking seems to be connected with fungus attack. Resistance of polyesters to hydrolytic attack can be improved by the use of additives such as diisocyanates, fungicides, and carbodiimides. The latter are most useful and apparently scavenge the carboxylic acid resulting from hydrolytic breakdown of the polyesters thereby preventing autocatalysis. Their ability to react with carboxylic acids under these mild conditions has been reported. Differences among polyesters not only in inherent resistance to hydrolysis but also in response to inhibition are found. As has been seen the early cracking can be eliminated by use of sulfur cures, or inclusion of a fungicide but only at the expense of reduced resistance at 158° F. Accelerated tests have been shown to correlate reasonably well with outdoor exposures. Results on unstressed specimens correlate with the 158° F test while the stressed specimens yield results similar to exposure at 95 per cent R.H. and 100° F. Not all linear polyesters are susceptible to hydrolysis such as is poly (ethylene propylene adipate). The terephthalate polyesters, poly (ethylene terephthalate) and poly(1,4 cyclohexane diearbinyl terephthalate), have been exposed in the form of thin films over water at 158° F for 14 months and over water at 95 per cent R.H. and 100° F for 23 months without visible signs of deterioration. They are apparently stabilized by the presence of the aromatic rings or steric factors or a combination of these. In conclusion it must be said that although some progress has been made in protecting polyester urethan elastomers from hydrolytic failure under tropical and high humidity conditions, the problem has only partially been solved.