Mesh size distributions (MSDs) of swollen cross-linked styrene copolymer networks have been measured by thermoporometry using CCl4 as a probe liquid. All numerical relationships required for the calculation of the MSD were established for both the liquid-to-solid and the solid-to-solid thermal transitions Of CCl4 and successfully validated on test samples. It was found that the polymer network, for both thermally and photo-cross-linked materials, was completely built in about 4 h of exposure. A clear correlation was established between the average mesh size of the swollen polymer network on one hand and the benzoyl peroxide groups content and swelling ratio on the other hand.