Intraparticle mass transfer coefficients were determined by experimental adsorption data of m-cresol, quinoline and 1-naphthol onto a silica gel on a finite bath. The equal area approximation method of nonlinear isotherm was proposed with the assumption that the internal mass transfer coefficient is constant and equal to average value in the operating region. In the region where a slope of a constant time line is less than that of the operating line(region I), the original nonlinear isotherm was approximated by a quadratic form, and in the inverse case(region II) the isotherm was approximated by a linear form. Thus approximate analytical solutions were obtained. The present experimental conditions were in the region II, and the present approximate solution was in a good agreement with experimental results.