Cationic polymerization of glycidol ((hydroxymethyl)oxirane) may involve two competing propagation mechanisms : active chain end (ACE) mechanism (nucleophilic attack of the monomer on the tertiary oxonium ion active species) and activated monomer (AM) mechanism (an attack of the hydroxyl group of the polymer on the protonated monomer). The first mechanism should lead to polymers containing exclusively primary hydroxyl groups. Propagation by the AM mechanism leads on the other hand to polymers containing both primary and secondary (mostly) hydroxyl groups, depending on the direction of the opening of the protonated oxirane ring. Analysis of the microstructure of polymers of glycidol of relatively high molecular weights (($) over bar M(n) = 10(3)-10(4)) by C-13 NMR spectroscopy as well as by Si-29 NMR spectroscopy of the silylated samples shows the presence of the secondary hydroxyl groups. The significant fraction (up to 50%) of the secondary hydroxyl groups in the polymer indicates the important contribution of the AM mechanism of propagation.