Monofunctional polydimethylsiloxanes with a dimethylsilyl or a dimethylsilylvinyl end-group were reacted with respectively 1,3,5,7-tetramethyltetravinylcyclotetrasiloxane and 1,3,5,7-tetramethylcycloterasiloxane. The hydrosilylation reaction, carried out in bulk and in the toluene solution, were followed by FTi.r. spectrometry and the star-branched samples formed analysed by SEC-LS. In the second case, for PDMS silylvinylic function at the chain-end, the reaction is noticeably faster and the final conversions reached are higher than in the first case. The principal parameter which limits the maximum conversion is the concentration of functional groups in the medium.