The effect of dilution on random living copolymerization of vinyl- and divinyl monomers with fast initiation and slow propagation was simulated and compared with the experiments. Two Monte Carlo simulation methods have been used: one, purely statistical, based on the Flory-Stockmayer (FS) theory and the second using the dynamic lattice liquid model (DLL). The results were compared with experiments using atom transfer radical polymerization (ATRP), one of controlled/living radical polymerization methods. Molecular weights, polydispersities and the crosslinking/cyclization of macromolecules were analyzed as a function of conversion and time for various solvent concentrations and initiator crosslinker/monomer ratios. The results obtained by the DLL method are in good agreement with the experiment, showing an increase of conversion and time at gel point with dilution, i.e., with the increase of solvent concentration. The FS theory was found to be unable to reproduce even qualitatively the dilution effect on gelation. It is shown that intrachain crosslinking becomes more and more important with increasing solvent concentration, preventing macroscopic gelation at high dilutions, even if the number of crosslinks per primary chain is much greater than one (Flory criterion). DLL simulations give an insight into the mechanism of crosslinking processes near the gel point (leading to gelation) and/or microgel formation when solvent concentration is too high. At very high solvent concentration polymerization is slow and side effects like deactivation of radicals become important. Taking such effects into account, the simulation gives better agreement with the experimental data. (C) 2011 Elsevier Ltd. All rights reserved.