omparison analysis of linear least-square method and non-linear method of three widely used isotherms (Langmuir, Freundlich, and Redlich-Peterson) were examined to experiment with Remazol Red (C.I. 18221) onto pistachio nut shells at a solution temperature of 293 K. All three isotherm equations showed a better fit to the experimental equilibrium data. The results show that the Langmuir model shows the best fit compared to Freundlich and Redlich-Peterson models using the linear method. However, when using the nonlinear method the Freundlich and Langmuir models show the best fit compared to others. Redlich-Peterson isotherm is a special case of Langmuir isotherm when the Redlich-Peterson isotherm constant g was unity.