Experimental results for some properties (activity of water, change of enthalpy on mixing and diluting, compositions of coexisting liquid phases) of aqueous solutions of hydrophilic polymers poly(ethylene glycol) and dextran between 277 and 333 K are reported. The results are used to develop and test a semiempirical group contribution expression for the excess Gibbs energy of diluted aqueous polymer solutions. The model is applied to correlate some of the new experimental data, while the remaining experimental results are used to test its potential for predicting the influence of polymer molecular weight and temperature on the phase behavior of aqueous two-phase systems - a field where most previously known methods fail.