The diffusion of methanol in PMMA and dodecane in PS has been studied by in situ dynamic observations using light microscopy. A specially constructed cell enabled accurate monitoring of the diffusion fronts as they advance into the polymer matrices. It is shown that the diffusion rate at the early stages of the process does not follow the rate predicted by square root from time behaviour and that swelling is also delayed. Analysis of the results indicated that the following equation gives an accurate correlation with the experimental data: x = b(t(1/2) -(1 - e(-t1/2))). It was also observed that voids can be filled up by the penetrant, representing diffusion against the concentration gradient and thus indicating that concentration driven mechanisms are not applicable to the systems studied in this work.