Abstract: 1. Radiolysis of the system TBP-DBBP-C6H6 and the dependence of GDBp+MBP on the composition of the system were investigated.Three solutions of TBP(32P)-C6H6 were prepared. The mole fractions of benzene in these solutions were 0.826, 0.785, 0.916 and the mol ratios of C6H6 to TBP were 4.8, 3.72, 10.98 respectively. Different amounts of DBBP were added to these solutions, and then they were irradiated. The results give evidence that the apparent action of DBBP is dilution, and the protective ability of benzene is depressed correspondingly. While treating DBBP as a diluting agent to the system C6H6-TBP, G value of acid products from the excited state of second type have been found to be 0.95±0.11. This state contributes about 43% of the excited states. It probably comes from the excitation of (?)P=0 bond to "a higher singlet state. The protection of phenyl group on this state shows a linear relationship between the G value of DBP-MBP (Gi) versus mol fraction of benzene, i.e.2Gi = X(1-x).The kinetic study shows that collision mechanism predominates in the process of energy transfer, especially at lower concentration.2. DBPP has a twofold protective effect on excited TBP, i.e.the effect of phos-phoryl group and that of phenyl group. The inhibition effect of phosphoryl group issimilar to that of DBBP and that of phenyl group is the same as benzene. It also shows a linear relationship between 2Gi and mol fraction of DBPP. i.'e.2Gi=2G0(1-x).3. Under gamma radiation, TBP may form three excited and ionic species at least, i.e.triplet state 29±3%, singlet state 43±4% and TBP+ 28±3%.