4H-1,3,2-Benzodioxasilines represent a class of monomers, which are suitable for the recently developed concept of twin polymerization. Relating to the prediction of quantum chemical calculations we are now able to confirm the results for the first step of the reaction: the selective ring opening of 4H-1,3,2-benzodioxasilines. An electrophilic attack of iodotrimethylsilane leads firstly to a bond cleavage at the oxymethylene group and not at the more stable Si–O–aryl bond, yielding ring-opened species.