Knowledge of the molecular properties, e.g. the molar mass, of lignin are of central importance for industrial applications, since thermal and mechanical characteristics are governed by these. Here, a method for absolute determination of weight-average molar mass of kraft lignin, based on diffusion constants gained from PFG-NMR calibrated with lignin fractions characterized by MALDI-TOF-MS, is presented. The NMR method is compared to polystyrene calibration, and size exclusion chromatography with tetrahydrofuran as mobile phase.