A method is described for the determination of the number average functionality and functionality distribution of polyether polyols based on the measured intensities of relevant end groups observed in C-13 NMR spectra. Longitudinal (T1) relaxation time measurements on glycerol-, trimethylolpropane-, and pentaerythritol-initiated copolymers of propylene oxide and ethylene oxide show that the slowest relaxing moiety that needs to be quantified is the vinyl CH2 = group of the unsaturated side product of the propoxylation reaction. A chromium(III)-containing relaxation reagent allows the analysis to be speeded up by decreasing the relaxation times. The relative precision of the functionality measurement is ca. +/-1%, while determinations of the mole percentage levels of species with different functionalities can be carried out with a precision better than +/-3 mol%.