The cationic copolymerization of ε-caprolactone with ethylene oxide (EO) under the conditions of activated monomer polymerization, that is, with a low-molecular-weight diol as an initiator and BF3 etherate as a catalyst, was studied. The copolymerization was conducted with incremental feeding of the EO comonomer. 1H NMR analysis of samples isolated at different stages of the copolymerization indicated that the average composition of the copolymer was nearly constant over the course of the copolymerization. Matrix-assisted laser desorption/ionization time-of-flight spectra of the products revealed, however, that for the same degree of polymerization, macromolecules containing different numbers of EO units were present. The observed distribution was compared with the simulated one. A good agreement between the observed and simulated spectra was obtained. This indicated that, even when the optimum conditions for the formation of a uniform copolymer were created, the individual macromolecules differed in composition because of the statistical character of the copolymerization.