Cyanex 302 and Cyanex 301 are recently available solvent-extraction reagents, the major components of which are the mono- and dithiosubstituted analogs of bis(2,4,4-trimethylpentyl)phosphinic acid, respectively. Sulfur substitution in the phosphinic acid increases the acidity of the extractants, and makes them particularly suited to the extraction of soft Lewis acids. Sulfur-containing dialkylphosphoric-acid extractants previously developed, however, are known to be very susceptible to oxidation and decomposition. To evaluate the extents of reagent decomposition which could reasonably be expected under normal plant operating conditions, this study examines the hydrolytic, chemical and thermal stabilities of Cyanex 302 and Cyanex 301 when subjected to extreme experimental conditions and prolonged periods of exposure to sulfuric and nitric acids. It is observed that concentrated sulfuric acid has no long-term detrimental effect on either of these reagents, but that the strongly oxidizing nature of nitric acid causes rapid destruction of the reagents, even at low acid strengths and for short periods of time.