Electrophoretic NMR has been applied for the first time to measure electroosmotic drag coefficients K-drag in polymer electrolyte membranes. Theoretical and experimental details of the method are discussed and measurements of K-drag as a function of water content and temperature are reported for Nafion(R) 117 and sulfonated PEEKK. For a given water content n = [H2O]/[SO3H], the values for sulfonated PEEKK are lower than for Nafion, but for the water contents of the highest proton conductivities observed (n = 20 for Nafion n = 40 for sulfonated PEEKK), the results are similar for both polymers (K-drag = 2.6 for Nafion and K-drag = 3.1 for sulfonated PEEKK). A hydrodynamic model equation with the rate of proton transfer processes and the water-polymer interaction as parameters is used for the interpretation of the data.