The phase behavior of poly(N-tertbutylacrylamide-co-acrylamide) (PNTBAM) in pure water and mixture of water-methanol is studied at different temperatures. The different compositions of PNTBAM are prepared by free-radical polymerization technique and their phase behavior is studied by turbidimetry. The effects of copolymer and solvent composition on the phase behavior of the copolymers are discussed. It has been suggested that the inhomogenities in polymer chains are responsible for lowering the rate of phase transition by increasing the N-tertbutylacrylamide (NTBAM) and methanol contents in copolymer and mixture, respectively. For the first time we have revealed that there are second-order binary interactions in the water-methanol which are dominant in the special range of copolymer composition. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polyrn Phys 47: 455-462, 2009