A nonlinear optimization program was used to fit the model equation to the experimental equilibrium data of a thorium-nitrate/nitric-acid-1-M di-n-heptyl sulfoxide (DHPSO) 1,1,2-trichloroethane (TCE) system successfully. The extraction equilibrium curves for this system obey Langmuir type. The product of parameters A and K is the maximum extraction distribution coefficient that can be used to compare the extraction power of 1 M DHPSO-TCE with respect to Th(NO3)4 at various aqueous acidities.