A synthetic polymer forming a thermo-reversible coacervate in aqueous milieu was prepared in this study by radical copolymerization of N,N-dimethylacrylamide (DMAA) with N-phenylacrylamide (PA). Increased content of PA in the copolymer (DPA) led to an abrupt decrease in its cloud point (lower critical solution temperature, LCST) owing to an increase in hydrophobicity. The heat of transition (Delta H) for a 1.0 wt% aqueous solution of the copolymer at the LCST was determined to be in the range of 3-4 cal g(-1) by differential scanning calorimetry, considerably smaller than that obtained for a neutral amphiphilic polymer, such as poly(N-isopropylacrylamide), undergoing a steep dehydration at the LCST. This small value of Delta H suggests the incomplete dehydration of the polymer chain at the LCST. Indeed, microscopic observation revealed the formation of coacervate droplets in the aqueous solution of the copolymer, which is clear evidence of liquid-liquid phase separation. The concentration of the copolymer in the coacervate phase formed from 1.0 wt% aqueous solution of the copolymer at 37 degrees C is approximately 20 wt%. It is of interest that the formation and disappearance of the coacervate are completely thermo-reversible. To estimate the feasibility of applying this thermo-sensitive coacervate system to thermo-modulated aqueous two-phase separation, partitioning of model solutes from aqueous milieu to the coacervate was carried out. By increasing the temperature, preferential partitioning of Trypan Blue (3,3’-[(3,3’-dimethyl[1,1’-biphenyl]4,4’--diyl)bis(azo)]bis(5-amino-4-hydroxy-2,7-naphthalenedisulfonic acid) tetrasodium salt) from aqueous phase to coacervate phase was observed, whereas no partitioning of vitamin B-12 to coacervate phase took place. Consequently, the separation of Trypan Blue from the mixed solution of vitamin B-12 and Trypan Blue was achieved solely through the change in the environmental temperature. It may be feasible to apply such copolymers showing liquid-liquid phase separation that responds to temperature as the stationary phase in thermo-regulated liquid chromatography for the separation of water-soluble drugs and dyestuffs.