Biodegradable hydrogels of cross-linked polymethoxyethylaminophosphazenes (PMOEAPs) of various cross linking density and apparent subchain hydrophobicity were investigated by high-sensitivity differential scanning calorimetry and equilibrium swelling measurements. The volume phase transition of the hydrogels was found to be induced by salts of weak polybasic acids. The transition parameters were determined depending on the pH, phosphate concentration, cross-linking density, and apparent hydrophobicity of the gels. The transition enthalpy increased three times and reached 60 J g(-1) at the phosphate concentrations 5-100 mM. The transition temperature decreased by 60 degrees C when the pH changed from 6 to 8. A decrease in the transition temperature (by similar to 20 degrees C) was achieved due to incorporation of 9.4 mol % of some alkyl groups into the gel subchains. The classic theory of the collapse of polymer gels coupled with the data of protein science on hydration energetics for various molecular surfaces reproduces correctly thermodynamics of the collapse of PMOEAP hydrogels.