The enthalpy, Gibbs free energy, and entropy of mixing of polyacrylamide with agarose have been determined using thermodynamic cycles based on microcalorimetry and isothermal equilibrium sorption data. The enthalpies of solution in water have been measured, and water sorption isotherms have been obtained for films of polyacrylamide, agarose, and their mixtures in different ratios. The sorption data have been processed used a sorption model that takes into account the effect of polysaccharide chain stiffness on the change in the chemical potential of water upon the formation of a solution during sorption. The model relationships are shown to be in good agreement with the experimental isotherms over the entire range of variation of the polymer volume fraction and the relative vapor pressure. It has been found that the enthalpy of mixing of polyacrylamide and agarose is negative over the entire range of compositions. Mixtures containing less than 50 wt % agarose are thermodynamically incompatible and are characterized by a positive Gibbs energy of mixing, whereas Gibbs energies are negative for mixtures containing more than 50% agarose. The calculated values of the entropy of mixing are negative in the entire range of agarose to polyacrylamide ratios, thereby indicating ordering in this polymer mixture. The results of studying the thermodynamic compatibility of polyacrylamide and agarose are compared with the swelling behavior and mechanical properties of hydrogels based on semi-interpenetrating networks of these polymers.