We give a quantitative comparison of dielectric and viscoelastic relaxation of identical poly(oxypropylene) (POP) diols and triols with molar masses in the range 260-11 000 g/mol. The segmental relaxation probed in dielectric spectroscopy (DS) is slower by a factor of 8 +/- 1, while the conformational relaxation of the whole chain is faster by a factor of 2.5 +/- 0.5. The molar mass dependence of the conformational relaxation is close to the prediction of the Rouse model. POP diol with molar mass larger than 4000 g/L shows the influence of entanglement. A "fast" beta-relaxation with Arrhenius temperature dependence is observed in all samples by DS. Measurements made on very low molar mass samples show that hydroxyl end groups are involved in this relaxation.