Atmospheric pressure chemical ionization (negative ion) and electrospray ionization (positive and negative ion) mass spectrometry and nuclear magnetic resonance (1H NMR and 13C NMR) were used to characterize the prepolymer (i.e. uncrosslinked) poly(xylitol sebacate) (PXS), prepared by condensation of xylitol [X] and sebacic acid [S], and this was compared with poly(glycerol sebacate) (PGS), prepared by condensation of glycerol [G] and [S]. The PXS prepolymer, prepared at 118 ∘C/24 h, gave predominantly 1-acyl-[XS] and 1,5-diacyl-[SXS] species and smaller amounts of 2-acyl substitutions. At 130 ∘C/24 h an increase of the amount of the 1-acyl and 1,5-diacyl substitutions in the [(SX)n], [S(XS)n] and [(XS)nX] series was identified, as was found for the analogous PGS prepolymer ([(SG)n], [S(GS)n] and [(GS)nG] series). Novel tetrahydrofuranyl species were found in the PXS prepolymer. Further polymerization of the PXS prepolymer gave a gel whichwhen analysed using 13CNMRwas shown to mainly contain 1-acyl and 1,5-diacyl substitutions.