In this work, the degree of crosslinking on the volumetric contraction-expansion processes of hydrogels made of poly(N-isopropylacrylamide) (NIPA) (initial amount: C with varied amount (z) of crosslinking agent methylene-bis-acrylamide (BIS) in reference to most commonly used NIPA gel that was synthesized with C-m= 700 mM and z = 8.6 mM was investigated by applying our recently developed pycnometry. We focused on characteristic role of four polymeric NIPA residues directly bonded to a single BIS molecule by evaluating the total volume of gels per four NIPA residues directly bonded to a BIS molecule, plus associated water [v(sp)(gel)(NIPA)(bonded))(T)], and the corresponding number of water molecules per four NIPA residues [v(sp)(gel)(NIPA)(bonded))(T)]. We elucidated how these quantities characteristically contribute to changes in the volumetric contraction-expansion processes of hydrogels. A comparison of these quantities with the corresponding quantities for (NIPA)(unbonded) residues clearly revealed a significant structural difference between (NIPA)(bonded) and (NIPA)(unbounded). (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3177-3184, 2009