Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) allows mass measurements of large molecules such as those present in synthetic and natural macromolecules. We have used a self-calibrating method for the MALDI-TOF spectra of polymers, enabling us to obtain : accurate mass values. Poly(ethylene glycol) (PEG) samples having two different kinds of end groups were used in our work : (i) anionic PEG, H-(CH2-CH2-O)(n)-H; (ii) cationic PEG, H-(CH2-CH2-O)(n)-OH. The MALDI-TOF spectra recorded in reflected mode allowed unambiguous identification of the end groups present in these PEG samples. Five anionic PEG samples with very narrow molecular weight distributions (MWD) were studied. The MALDI-TOF spectra of PEGs were recorded in both linear and reflected modes. Due to the high sensitivity and the highly linear response of HIMAS (a microchannel detector equipped with a photomultiplier), measurements of MALDI-TOF spectra were used in our work. to estimate the MW and MWD of the PEG samples. Our results show that the molecular weight estimates provided by MALDI-TOF measurements agree with the values obtained by conventional techniques such as GPC, osmometry, and viscometry.