Hydrogen-bonded interpolymer complexes between poly(N-isopropylacrylamide) (pNIPA) and poly(methacrylic acid) (pX1AA) were studied in aqueous solutions by means of high-sensitivity differential scanning calorimetry (HS-DSC), analytical ultracentrifugation, and light scattering. Composition of the complexes was varied by changing the pMAA/pNIPA molar ratio r and pH. Solubility of the pMAA/pNIPA complexes increased with increasing the content of pMAA in the complex and decreased with decreasing pH. According to HS-DSC, the phase transition behavior of pNIPA upon heating is significantly modified in complexes with pMAA. Thus, decreasing pH from pH 7 to 3 or increasing the content of pMAA decreases the phase transition temperature of the complexes by 4-5degreesC with respect to the lower critical solution temperature (LCST) of pure pNIPA. At the same time, the enthalpy of the transition decreases down to zero and the transition width increases by more than 1 order of magnitude. Thermograms of the soluble complexes (4.3 < pH < 6.3, r = 2.7) show an additional cooperative transition preceding the phase separation upon heating. This transition is accompanied by increasing the sedimentation coefficient of the complexes and decreasing their mean hydrodynamic diameter. Experimental data suggest that the pMAA/pNIPA complexes are able to fold cooperatively to a compact structure without a loss of solubility at temperatures below the LCST of pNIPA.