In situ fluorescence technique was used to study the free-radical crosslinking copolymerization of styrene and divinylbenzene in bulk. Pyrene (Py) was used as a fluorescence probe for the real time gelation experiments. The critical times, t(r) required for a dramatic increase in Py intensity, I-p, corresponding to the onset of the gel effect were determined. The spikes in I,curves were also observed at high crosslinker densities. These spikes were attributed to the existence of "critical opalescence" during the gel formation process where two different gel phases having two different concentrations coexist. to values at which the critical opalescence is occurred are recorded in the close vicinity of the t(r) points. (C) 1999 Elsevier Science Ltd. All rights reserved.