When there is a saturated homogeneous ternary mixture, composed by two immiscible liquids and an amphiphilic one, it is possible to prepare a second one starting from the first. The compositions of the two mixtures are uniquely determined if their solubility curve is known. In this work, one describes a new procedure to determine these compositions. The procedure is applied to a ternary system known in literature: water + n-propanol + n-butanol at 294.15 K and atmospheric pressure. Comparing the results obtained by the new procedure to those determined by measuring the refractive index one concludes that the two analysis methods have similar accuracies.