Kinetic models are often used to analyze batch flotation results. Two parameters, R-infinity (ultimate recovery) and k (first-order rate constant) are obtained from the model fit to an experimental recovery-time curve. Comparison of either R-infinity or k obtained under different conditions can often be misleading. In order to overcome this problem, a modified flotation rate constant, K-m, is introduced. It is defined as the product of R-infinity and k, which is the slope of the tangent of the recovery-time curve at zero flotation time: K-M = R-infinity.k Consequently, a parameter measuring the degree of selectivity or a selectivity index can be introduced among the different minerals presented in a flotation system. Two examples of a typical nickel sulphide ore flotation are given to illustrate the calculation of the modified flotation rate constant and selectivity index.;the modified rate constant and selectivity index are useful tools for comparative evaluations of various operating parameters affecting flotation processes.