A neutral extractant, tetra(tert-butyl)-calix[4]bis[(4,4'-dimethyl-1,2-phenelyene)-crown-6] (TTCalixB6), was prepared. The extraction equilibrium of Cs(I), Na(I), K(I), and Rb(I) and some coexisting metals Sr(II), Ba(II), Fe(III), and Ru(III) with TTCalixB6 was investigated in the range of 0.42-6.0 M HNO3. The effects of two types of organic diluents o-nitroanisole, an electron donor, and chloroform, an electron acceptor, on the extraction of TTCalixB6 were examined. It showed a high extraction ability for Cs(I) in HNO3/o-nitroanisole over others in HNO3/chloroform. The association of TTCalixB6 with HNO3 and chloroform through hydrogen bonding was the reason why Cs(I) extraction decreased in HNO3/chloroform. The composition of Cs(I)-extracted species was a 1:1 type of the complex, CsNO3 center dot TTCalixB6, determined by electrospray ionization mass spectrometry. More than 98% of Cs(I) alone with less than 5% of Rb(I) were extracted with TTCalixB6/o-nitroanisole based on a separation framework. The thermodynamic parameters Delta H degrees, Delta G degrees, and Delta S degrees and some fundamental chemical engineering data of TTCalixB6 extraction were obtained. TTCalixB6/o-nitroanisole is promising to apply in Cs(I) removal from a high-level liquid waste.