The effects of variables on the rate of Purex solvent deterioration were investigated with the emphasis on those deterioration products which cannot be removed by carbonate washing. The deterioration rate was found to be directly proportional to the acid and nitrate ion concentrations, and proportional to the square root of the concentration of nitrite ion. Other observations on the effects of temperature, relative merits of brand name solvents, and the effects of radiation are included. A method of calculating the equilibrium level of the solvent deterioration products based on study conclusions and various estimates is outlined. (J.R.D.)