The radiolysis of tri-n-butyl phosphate (TBP) is of interest for several reasons. For one thing, radiation damage to this material is of technical concern, since it is the active ingredient of certain solvent-extraction processes which take place in the presence of strong radiation fields. Second, it represents a substituted hydrocarbon of low volatility convenient for the study of radiation effects on substituted hydrocarbons. The factors in this radiolysis which are here discussed are (1) the production rate and composition of the radiolysis gases; (2) the production rate of monoand di-n-butyl phosphates; (3) the production rate of polymeric material; and (4) the amount of TBP which disappears by all processes-i.e., the value of G-M. All rates are reported as G values (molecules of product formed per 100 ev of energy absorbed in the system). Since the completion of this study (1), other reports on the radiolysis of TBP have appeared (2, 3); however, this report presents a rather more complete study than the other two, and the comparison of data is of interest.