Accurate kinetics equations, which can be applied to a square and to a two-step gas-separation cascade composed of stages with a large separation factor, are derived from the exact conservation of matter in the unsteady state. The derivation is based on the assumptions that flow rates and holdups are independent of time and that the second derivative of the assay with respect to time can be neglected. If two or three additional assumptions, including the important one that the separation factor is nearly equal to unity, are added to those above, the author’s equations reduce to Cohen’s kinetics equations. If a square cascade with eight stages composed of separators having a separation factor of 1.1 is supposed to be operated in total reflux, the results of the calculations disclose that the assays and the 98% equilibrium times obtained from the conventional equations are overestimated by ∼12 and ∼10%, respectively, compared with those obtained from the author’s equations. The author’s kinetics equations promise to be useful for analyzing the kinetics of a square cascade with a large separation factor such as a centrifuge.