The radiation-induced copolymerization of octamethylcyclotetrasiloxane (D4) with hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane with decamethylcyclopentasiloxane (D5) was used to examine the relative reactivities between the three cyclic monomers, under super-dry conditions. The copolymerization results have shown very similar reactivities of the three monomers. These results were in agreement with previously reported monomer reactivities and homopolymerization activation energies, as well as with the production of similar quantities of cyclic products. A single reaction mechanism for the polymerization of D3, D4 and D5 is proposed for both the propagation and the backbiting reactions. This mechanism assumes that backbiting (giving D6, D5 and D4 but not D3) involves, as intermediates, silicenium ions solvated by nearby oxygens of their own chain, and propagation by a much more reactive unsolvated (unpaired) silicenium ion in lower concentration.