A comparison of the linear least-squares method and a trial-and-error non-linear method of three widely used isotherms (the Langmuir, Freundlich, and Redlich-Peterson) were examined in an experiment using lead ion sorption onto peat at different temperatures. The four Langmuir linear equations are discussed. Langmuir isotherm parameters obtained from the four Langmuir linear equations using the linear method differed but they were the same when using the non-linear method. Langmuir-1 is the most popular linear form, and it had the highest coefficient of determination compared with the other Langmuir linear equations. The Redlich-Peterson and Langmuir isotherms produced higher coefficients of determination. There are two separate regions depicting the Freundlich isotherm. The results show that that non-linear method may be a better way to obtain the parameters.