Selectivity in intermacromolecular complex formation has been studied for some three component systems involving poly (ethylene imine) (PEI), poly (ethylene oxide) (PEO) and poly (acrylic acid) (PAA). The stability constant and related thermodynamic parameters of these complexes were determined at several temperatures. The entropy and enthalpy changes of the systems with temperature have been interpreted in terms of destabilization of the various interacting forces involved in complex formation as a result of pH of the medium.