In this paper, an ideal swelling curve is demonstrated based on Kirchheim's free volume distribution theory. When a series of solvents with different molar volumes are used to measure the swelling degrees of a polymer, the ideal swelling curve can give a good explanation for the polymeric swelling if the range of molar volumes is broad enough. The precondition is that the polymeric free volume distribution becomes the deciding factor in swelling. This requires the solvents to have strong affinities for the polymer and for the swelling degrees to be small. The ideal swelling curve has been tested with a series of swelling experiments that involved two polyimides (HQDPA-DMMDA and ODPA-DMMDA) and four polar solvents (water, MeOH, EtOH, and i-PrOH). The ideal curve approach is able to interpret the abnormal phenomenon discovered by Jonquieres et al. when they observed the swelling degrees of some cross-linked unsaturated polyesthers in polar alcohols.(c) 2005 Elsevier B.V. All rights reserved.