We used deliberately calibrated ultrafiltration membranes to analyze the molar mass distribution (MMD) of kraft lignin in alb aline solution. The membranes were first tested with probe macromolecules to obtain sieving curves at the same conditions as the lignin analysis. Using a restricted transport model, we calculated the average effective membrane pore sizes. The results were different from the nominal cut-off values. The MMD of a lignin sample at pH 13 was analyzed by separating the lignin solution into five fractions; the MMD using the calibrated cut-off values was confirmed by gel permeation chromatography.