Polyazeotropy in binary systems is a singular case of vapor-liquid equilibrium where at a given temperature or pressure various azeotropes of alternating deviations from ideality are observed. These deviations are generally caused by liquid-phase nonidealities, but in associated systems and even at low pressures, the vapor-phase nonidealities are remarkable. Associated systems are analyzed using chemical theory, where associated species are considered to be products of polymerization reactions; in those systems, azeotropy can be achieved without the usual requirement of equal composition in the phases. The purpose of this work is to show how the combination of nonidealities in both liquid and vapor phases can induce polyazeotropy and to establish its thermodynamical conditions.