We analyze a polymerization-induced phase separation taking place in solutions of a polymer dissolved in a vinyl monomer undergoing an isothermal free-radical polymerization. The selected system was a solution of polyisobutylene (PIB) in isobornyl methacrylate (IBoMA). Cloud-point curves for PIBs of different molar masses were fitted with the Flory-Huggins equation stated for a 3-component system. Phase separation was analyzed for two different PIBs at off-critical compositions. In a comparison of experimental values of the volume fraction of dispersed phase with those predicted by the coexistence curves, it was found that the system evolved close to equilibrium during an initial stage. But phase separation stopped at intermediate monomer conversions as assessed by scanning electron micrographs of partially converted materials. The evolution of the size distribution of dispersed domains was obtained from the fitting of light scattering (LS) spectra. Average sizes predicted by LS and measured in scanning electron micrographs were in rough agreement.