Many papers report that accuracy of the Redlich-Peterson (R-P) isotherm equation (three parameters) is higher than those of Langmuir and Freundlich isotherm equations (two parameters). This paper first explains why the accuracy of the R-P isotherm equation is equal to or higher than that of two parameters isotherm equations. This study suggests a new exponential linear equation with exponent variable a, and solves the most suitable a value with the aid of Microsoft Excel and Sigma Plot 9.0. This exponential method is quick, simple, and accurate for fitting the R-P isotherm equation to experimental data sets of adsorption systems compared with a logarithmical linear form frequently used previously. This investigation prepares four kinds of pistachio shell activated carbons with various doses of NaOH for studying isotherm equilibrium adsorptions of three dyes. The R-P isotherm equation best fit adsorption system data sets studied in this work. (C) 2010 Published by Elsevier B.V.