The Maxwell equal area rule is a basic construct for determining saturated phase volumes from equations of state. The fundamental idea is that for a pure substance a horizontal line drawn through the trace of an isotherm, which equalizes the van deu Waals loops area above and below the line, intersects the isotherm at the saturated volumes. Another basic construct exists for determining the phase compositions in a binary mixture by establishing the tangent line to the total Gibbs energy curve plotted against composition. We demonstrate that finding the tangent also reduces to an equal area construction for the derivative of the total Gibbs energy plotted against composition. An efficient algorithm suggested effects such equal area determinations. The trial-and-error procedure normally converges in one to three steps to the con ect solution given any reasonable starting guess. Implementation. using modem calculators with graphics capabilities is simple and straight forward.