Swelling behavior of polyacrylamide (PAAm) and polyacrylamide-co-poly- acrylic acid (PAAm-co-PAAc) gels was investigated in aqueous solutions of monodis- perse PAAms with molecular weights (M V w) ranging from 1.5 1 10 3 to 5 1 10 6 g/mol. The volume of the gels decreases as the PAAm concentration in the external solution increases. This decrease becomes more pronounced as the molecular weight of PAAm increases. The classical Flory-Huggins (FH) theory correctly predicts the swelling behavior of nonionic PAAm gels in PAAm solutions. The polymer-polymer interaction parameter x23 was found to decrease as the molecular weight of PAAm increases. The swelling behavior of PAAm-co-PAAc gels in PAAm solutions deviates from the predictions of the FH theory. This is probably due to the change of the ionization degree of AAc units depending on the polymer concentration in the external solution. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1313-1320, 1998