We have successfully used size selective P(N-Isopropyl acryl amide-co-Acrylic acid) gels to concentrate dilute aqueous solutions of high molecular weight Bovine serum albumin, dextran and poly vinyl pyrrolidone. Concentration factor of 2.4 with 96% efficiency was obtained for dilute solutlun while for semi dilute solution it was 1.77 with 81 % efficiency. Desired pore size of the gel is obtained by changing the concentration of monomer or cross linker while changed LCST is possible by modifying copolymer composition / change in pH of the aqueous environment. The resultant inexpensive and feasible under mild conditions separation process is alternative to ultrafilteration.