Poly(ethylaminophosphazene) (PEAP) in salt-free aqueous solution does not reveal thermoresponsive properties but acquires thermoresponsivity upon addition of various poly(protic acid)s. Energetics of the salt-induced phase separation transition of the PEAP solutions upon heating was investigated by high-sensitivity differential scanning calorimetry at pH 3.5. At low concentrations of poly(carboxylic acid)s (10-100 mM), the transition temperature and enthalpy vary from 90 to 25 degrees C and from 1.5 to 35 J g(-1), respectively, depending on the pK(a1) and the apparent hydrophobicity of the acids. Dependences of the transition parameters (T-v, Delta(t)h, and Delta(t)c(p)) on the citrate buffer concentration were obtained. The binding curve of citrate anions to PEAP was derived. High concentrations of sulfate anions (0.1-0.8 M) induced a marginal phase transition at high temperatures. A mechanism of the PEAP thermoresponsivity is proposed. The Okado-Tanaka model for cooperative hydration of macromolecules is used to quantitatively describe the energetics of the thermoresponsive behavior of PEAP.