A research report resulting from a three-year study on the stability of cellulose ethers, conducted at the Mellon Institute of Research, Carnegie Mellon University. The authors describe the chemical character of cellulose ethers as a general class of polymer and establish an approximate ranking of the stability of each generic chemical subclass. Cellulose ethers have attracted considerable interest among conservation professionals, but only limited information has been available concerning their potential long-term thermal and photochemical stability. This study reviews the physical and chemical properties of cellulose ethers as reported in the technical literature and presents laboratory research to support the ranking of eight generic classes of cellulose ethers with respect to thermally-induced discoloration, weight loss, loss of polymerization, and formation of peroxides. In the course of this research, individual commercial products of the same generic class were not compared or evaluated, nor was their effectiveness judged for use in conservation. However, a number of simple test methods are described that may be used by conservators to compare the relative stability of specific commercial products. -- AATA