Abstract: This article describes the use of derivatization as an efficient method for the determination of the nature of end groups in poly(ethylene-co-propylene) glycols. These data are helpful in calculating their composition at the molecular level with the aid of MALDI mass spectrometry. For derivatization, formylation is shown to be the most suitable. Knowledge of the nature of end groups allows an easy calculation of composition of the copolymers at the molecular level on the basis of MALDI mass spectra of underivatized and derivatized copolymers. It is underlined that derivatization is particularly useful in the investigation of minor components present in the samples. © 2012 Copyright Taylor and Francis Group, LLC.