The influence of the ionization efficiency on the measured copolymer sequence distribution is presented. Large differences in ionization efficiency were observed for mixtures of homopolyesters containing dipropoxylated bisphenol A/adipic acid and dipropoxylated bisphenol A/isophthalic acid and the corresponding copolyester dipropoxylated bisphenol A/isophthalic acid/adipic acid. The adipic acid structure has a higher affinity for the sodium cation, which results in more intense peaks for adipic acid containing oligomers. Relative sodium affinities of the oligomers were found to increase with an increasing number of acid end groups in favor of adipic acid containing oligomers. The ESI response of the oligomers depends on the polymer concentration in the sprayed mixture. This makes it impossible to correct for the ionization efficiency necessary for copolymer analysis. If differences in ionization efficiency are not corrected, the ion intensities in the copolymer mass spectra will show large deviations from the real composition and no conclusion can be drawn about the chemical (in)homogeneity of the MWD nor the random or block structure of the copolymer. This will also be valid for other cationization techniques like MALDI and FAB.