Photon transmission experiments were performed using a UV-visible (UVV) spectrometer during the swelling of polyacrylamide (PAAm) gets. These gels were prepared from acrylamide (AAm) with various N,N ' -methylenebisacrylamide (Bis) contents by free-radical crosslinking copolymerization (FCC) in water and dried before use for swelling experiments. Transmitted light intensity, I-tr increased at very early times when PAAm gels are immersed in water, then decreased continuously as swelling time is increased. Decrease in I-tr was attributed to the increase in the scattered light intensity, I-sc which may originate from the contrast between 'frozen blob clusters' and holes in the swelling gel. Decrease in I-tr was modelled using the Li-Tanaka equation from which time constants, tau (1), and collective diffusion coefficients, D-0 were determined for various Bis content PAAm gels. tau (1) and D-0 were found to be strongly correlated with the wavelength, lambda of I-tr and the Bis content of the gel samples. The correlation between lambda and tau (1) and D-0 predicts the existence of 'frozen blob clusters' in PAAm gels. (C) 2001 Elsevier Science Ltd. All rights reserved.