We report on a sensitive and reliable ac technique to measure the Seebeck coefficientSof materials, in particular of highTcsuperconductors. The small temperature difference between the ends of the sample allows structure in theS(T) curve to be observed. This technique avoids the creation of a component proportional todS/dTin the measured signal. Such a component has in the past led to erroneous conclusions regarding the high‐temperature superconductor YBCO. A mathematical analysis shows the origin of this component and how it can be avoided. These predictions are confirmed by experiments on a YBCO single crystal.