The binary sorption of copper and nickel ions onto peat has been studied. Equilibrium isotherms have been determined for three different copper : nickel molar concentration ratios. The experimental results have been analysed using an extended multicomponent Langmuir equation with a competition term but the correlation between the experimental and the predicted values was poor. An interaction factor was incorporated into the extended Langmuir equation and a significant improvement in correlation was obtained. The final model involved deriving an expression to correlate the interaction factor with surface loading and these correlations produced coefficients greater than 0.995 for all the isotherms. An F-test on the variable interaction factor model developed in this paper confirmed that it produces a significant improvement in correlating isotherm equilibria data.