Solvent extraction studies of99TcO4−and its nonradioactive congener ReO4−into 1,3-diisopropylbenzene and several other organic solvents containing the 0.200 M Aliquat-336 nitrate (which is predominantly methyltrioctylammonium nitrate) from a variety of aqueous solutions led to the following conclusions: (i) ReO4−is a suitable solvent extraction surrogate for99TcO4−under conditions that model nuclear waste streams at the DOE Hanford site; (ii) distribution ratios (D values) greater than 100 can be achieved for TcO4−extraction from aqueous solutions containing a mixture of 1 M NaOH and 1.5 M NaNO; (iii) the addition of 1.5 M NaNO2or 0.14 M K2CrO4to the aqueous phase containing a mixture of 1 M NaOH and 1.5 M NaNO3does not significantly affect D(TcO4−); (iv) D(TcO4−) values were larger by a factor of 3-5 when ketone solvents with flash points above 60°C were substituted for 1,3-diisopropylbenzene. Initial and final concentrations of TcO4−- in the organic and aqueous phases were determined by γ counting of solutions of99TcO4−that contained tracer amounts of95mTcO4−(0.204 MeV). Initial and final concentrations of ReO4−in the aqueous phase were determined by ICP-AES.