The decarboxylation upon oligomerization of propylene carbonate in the presence of the bisphenol A/KHCO3 initiating system was studied by means of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Five different series of peaks appeared in the MALDI-TOF MS spectra of the oligomerization mixture. In addition to the chain-extension reaction with the propylene oxide units, the oligomers of all of the propylene oxide units and oligomers containing carbonate end groups condensed. The product of the oligomerization reaction was hydrolyzed under alkaline conditions. It was found that hydrolyzed oligomers do not carry carbonate groups. The results of the MS studies agreed well with those obtained by H-1 and C-13 NMR spectroscopies and size-exclusion chromatography (SEC). On the basis of the results of the present investigation, a mechanism for the oligomerization is also proposed.