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Summary This work describes long-term thermal stability limits of water-soluble polymers under anaerobic conditions. Polymers investigated included polyacrylamide, xanthan, scleroglucan, cellulose sulfate, and a heteropolysaccharide of unknown structure. The primary mechanism of polyacrylamide degradation was found to be amide group hydrolysis. Interaction between hydrolyzed polyacrylamide and divalent metal ions present in solution caused significant losses in solution viscosity, and phase Separation ultimately occurred in extreme conditions of high degrees of hydrolysis or high concentrations of divalent ions. The rate of hydrolysis was found to depend mostly on temperature. At 50°C [122°F], the rate was quite slow and polyacrylamide solutions were stable for many months, evert in the presence of high concentrations of divalent ions. At 60 to 70°C [140 to 158°F], the rate of hydrolysis was moderate and the rate of viscosity loss depended on the precise temperature and divalent ion concentration. At 90 °C [194°F], hydrolysis was rapid and polyacrylamide solutions were stable to precipitation only when the divalent ion concentration was less than about 200 ppm. When the divalent ion concentration was zero, solution viscosity increased because of a further expansion of the polyelectrolyte coil. The stability of xanthan was determined primarily by temperature and was independent of divalent ions. Although performance varies from xanthan to xanthan, the useful limit was generally found to be < 70°C [< 158°F]. Viscosity retention was also found to be extremely shear-rate dependent. Other naturally occurring polymers exhibited variable performance. In alkaline brines, polyacrylamides were stable up to 90°C [194°F] for long periods of time, whereas xanthan was degraded at > 50°C [> 122°F].