Laser light scattering and differential scanning calorimetry measurements have been performed for aqueous solutions of thermosensitive linear copolymers of N-vinyleaprolactam and methacrylic acid of different composition. It was shown that the copolymers undergo a phase transition upon temperature increase in neutral and basic solutions. The enthalpy of the phase transition sharply decreases with the increase of methacrylic acid fraction, as shown by differential scanning calorimetry. The copolymers demonstrate pH-sensitive properties as well: intermacromolecular aggregation takes place in acidic media. FTIR spectroscopy measurements show that the aggregation is due to formation of insoluble macromolecular complexes. It was found that such complexes are also formed in the mixtures of homopolymers of poly(N-vinyleaprolactam) with poly(methacrylic acid). Effect of pH on thermosensitive properties of the copolymers is discussed. In weak acidic media there are narrow pH intervals close to pH of aggregation in which thermosensitive properties of the copolymers diminish considerably and the scattering intensity from the solutions at high temperatures is much less than at basic pH. Also, under these conditions the enthalpy of phase transition decreases significantly. The effect of a similarly charged surfactant (sodium dodecyl sulfate, SDS) on thermosensitive properties of the copolymer containing similar to40% of methacrylic acid is studied at pH 3 and 7. SDS solubilizes the copolymer, which is insoluble at pH 3; in the presence of SDS the copolymer is thermosensitive at both pH 3 and pH 7 at studied SDS concentrations; the transition temperature is higher than that of the copolymer in a surfactant-free solution. The enthalpy of the phase transition increases as SDS concentration in solution is increased.