Communication: Cationic polymerization of cyclohexyl vinyl ether (CHVE) within mesoporous, siliceous molecular sieves (MCM-41) yields novel MCM-41-polyCHVE host-guest hybrid materials, Initiation within the channel can be achieved by triphenylmethylium or bis(4-methoxy-phenyl)methylium silanolate ion pairs, as well as by mobile acidic silanols to a certain extent. Generation of the carbenium ions on MCM-41 is investigated by a special UV/VIS technique. The structure of poly(vinyl ether) generated within the channels of MCM-41 depends on the: mechanism of initiation as shown by means of solid state C-13(H-1) and Si-29(H-1) CP MAS NMR spectroscopy.