The purity of various commercially available sodium dodecyl sulfates was checked by gas chromatographic analysis of the fatty alcohols obtained from the acid hydrolysis of the alkyl sulfates. The content of tetradecyl sulfate in these samples was found to vary from 0 to 31% as impurity, while the content of the decyl sulfate homolog was ∼2% in all the samples. Accurate critical micelle concentration measurements are a sensitive means of detecting impurities, especially those of higher chain-lengths. These measurements have indicated the presence of large amounts of tetradecyl sulfate impurity in one of the “pure” samples.In the course of work on the determination of the binding of large amounts of various detergents to proteins (1), we have discovered that some of the commercially available “pure” sodium dodecyl sulfates fall far short of any conceivable standards of purity. Although one might expect rather small amounts of inorganic salts and organic compounds such as unsulfated alcohols, the major impurities found are the homologous sodium decyl sulfate and sodium tetradecyl sulfate.In this communication, we will report only the degree of impurity arising from decyl and tetradecyl sulfates in some of the commercially available samples of “ 99 1/2% pure” sodium dodecyl sulfates. Such large contents of tetradecyl sulfates have been found that even methods simpler than the more sensitive ones (i.e., gas chromatography of the fatty alcohols after acid hydrolysis) reveal their presence.