The formation of network structures associated with randomly cross-linked polydimethyl-siloxame chains is analyzed as a function of time. A cross-linking reaction between the methyl groups and the vinyl groups of vinylmethylsiloxane comonomeric units was induced by heating (150 degrees C) in the presence of a peroxide; transient states of gelation were observed by quenching samples in ice. The gel fraction, the swelling ratio, and an NMR structural parameter were the physical quantities used to investigate the network structures. The fraction of all vinyl groups that react in a time t is found to be proportional to t(1/2) in agreement with a mean field percolation treatment of systems of cross-linked chains. The maximum swelling effect was interpreted in terms of the packing of swollen Gaussian domains. The presence of silica aggregates did not perturb the network structures, as observed from their swelling properties.