The extraction of Fe3(+) and La3(+) with p-tert-butylcalix[8]arene octacarboxylic acid (H8L) and tributyl phosphate (TBP) was studied at 30 +/- 1 degreesC and ionic strength of 0.1. Fe3+ and La3+ are highly extractible with H8L; Fe3+ > La3+. The results showed that TBP had a significant synergetic effect on the extraction of Fe3+ by H8L. By the slope method, the extraction mechanism is given as Fe3+ + H8L(O) + 2TBP((O)) = (TBP)(2)Fe H5L(O) + 3H(+). The equilibrium constant of the extraction reaction was log Kex((S)) = 4.68 +/-0.12. However, TBP showed no synergetic effect on the extraction of La3+ with H8L.