Poly( ethylene glycol)s, with different molecular weights (4,000-20,000) and a monomethoxy-substituted poly(ethylene glycol)(molecular weight 5,000) have been quantitatively end group functionalized with bromo and amino groups. The bromination was carried out with thionyl bromide in toluene. H-1-NMR was used for the determination of the degree of substitution by monitoring the characteristic signals for the - CH2Br protons in relation to the signals from the remaining hydroxyl groups. Under the conditions used, qualitative substitution was obtained after repeating the bromination reaction. The results were in agreement with elemental analysis. The bromo end group could be quantitatively converted into amino groups by reaction in absolute ethanol with excess of ammonia or ethylenediamine. Also in this case, H-1-NMR was used to determine the quantitative substitution by the characteristic signals for the aminomethyl protons and the simultaneous disappearance of the bromomethyl proton signals.