The fundamental equations for calculating the extent of separation in binary elution chromatographic systems are derived. At a specific instant of time, the elution curve in such systems is characterized by two quantities: the optimum cut-point location and the optimum extent of separation. Equations relating the optimum extent of separation to the resolution and to the extent of separation for a single equilibrium stage are given. A number of figures obtained from computer examples illustrate the use of the theoretical results, A new type of component detector—a mobile detector—is proposed.