Optical transmission experiments were performed using a UV-visible spectrometer during the swelling of polyacrylamide (PAAm) gels. These gels were prepared from acrylamide with N,N'-methylenebisacrylamide at various onset temperatures (T-on) by free radical crosslinking copolymerization in water and dried before use for swelling experiments. Transmitted light intensity (I-tr) from the gels increased during early stages when PAAm gels were immersed in water at room temperature and then decreased continuously as swelling time was increased. Decrease in I-tr was attributed to the increase in the scattered light intensity 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 the PAAm gels prepared at different onset temperatures. tau(1) and D-0 were found to be dependent on T-on. The correlation between T-on and tau(1) and D-0 was explained in terms of the size of microgels formed during gelation. (C) 2003 Society of Chemical Industry.