The in situ, real-time, photon transmission technique was used to monitor the free-radical crosslinking copolymerization (FCC) of acrylamide (AAm) and N,N'-methylenebisacrylamide (Bis). Gelation experiments were performed with various Bis contents at various wavelengths. It was observed that the transmitted photon intensity, I-sc, decreased dramatically at a certain reaction time, which is attributed to the increase in scattered light intensity, I-sc, during the formation of microgels in the system. The increase in I-sc was modeled using Rayleigh's equation where the reaction time was found to be proportional to the volume of the microgels. In most cases, the change in I-sc was found to be inversely proportional to the 2.5 power of the wavelength, lambda, from which the size of the microgel particles were estimated during the FCC. (C) 2001 John Wiley & Sons, Inc.