For the first time, thermal field flow fractionation (ThFFF) was coupled online with H-1 NMR. ThFFF is one of the most important separation techniques for the analysis of large complex molecules and molecular assemblies. It provides information on molecular size and chemical composition in cases where multidetector setups are used. Multiple detection, however, is very complex. Using H-1 NMR as the sole detector offers a number of significant benefits First, NMR replaces the need to use multidetector setups. Second, the structure and the microstructure of the molecules as well as the chemical composition of the molecules can be recorded. As the consequence, selective and quantitative information can be directly obtained from the NMR chromatograms. Third, the molar mass distributions of all separated species can be derived directly from these NMR chromatograms. The new online ThFFF-NMR coupling was successfully applied to homopolymers and block copolymers of different molar masses and chemical compositions.