Abstract: The post-irradiation effects of 30% TBP-Kerosene-HNO3 systems have been investigated by means of the hafnium number test. It has been found that a delayed variation process of long duration of the hafnium numbers after irradiation occurs when the dose is greater than 5×106 rads. This variation process can be stopped by lowering the temperature to that of liquid nitrogen and it does not appear when the dose is lower than 4.59×105 rads. The general rule is that the values of the hafnium numbers drop rapidly after irradiation, then rise slowly, and finally get to an equilibrium value, The variation process can not be changed by sodium hydroxide solution and nitric acid solution scrub. These results suggest that the permanent and detrimental degradation products formed in irradiated 30% TBP-kerosene systems are secondary products. The effect of nitric acid concentration on the hafnium numbers has been studied. It has been found that there is a maximum at 0.5 N HNO 3 in aqueous phase for the starting values of the hafnium numbers as reported in most of the literatures.But under equilibrium conditions there is a minimum of the hafnium numbers at 0.5N HNO 3 and a maximum at 1N HNO 3. It is now universally accepted that the formation of the permanent and detrimental degradation products is the most favorable for 30% TBP-kerosene systems at 0.5N HNO3 in aqueous phase. Our experimental results show that this opinion is one-sided. In addition, we have examined effects of the methylation by diazomethane on the hafnium numbers. It has been found that the values of the hafnium numbers drop greatly after the methylation. It is indicated by the results of the methylation experiments that the detrimental degradation product is a substance containing active hydrogen.