A critical assessment of the use of a variable-order, power-law type surface reaction rate equation to correlate biosorption kinetics is presented. The general nth order rate equation with three adjustable parameters was fit to the kinetic data of lead uptake by inactivated cells of Rhodotorula glutinis using a genetic algorithm search method. The uptake process was fast, with apparent equilibrium reached in approximately 30 min. According to the Akaike information criterion test, the three-parameter nth order equation was superior to the much-used pseudo second order equation with two fitting parameters. However, the strong fit of the former equation resulted in unrealistic parameter estimates. Parametric sensitivity analysis indicated that the available kinetic data with only limited information content did not allow simultaneous identification of three unknown parameters. As a result, the three-parameter nth order equation was found to be overparameterized with highly correlated parameters. It was however possible to retrieve meaningful parameter estimates from the kinetic data when the number of fitting parameters was reduced from three to two.