The optical effects (fluorescence, anisotropy, and absorption) of kraft lignin solutions in three different kinds of solvents (0.1N NaOH, DMF, and pyridine) on the measurement of the weight-averaged molecular weight of lignin were investigated, and a correction procedure for these optical effects in the determination of the weight-averaged molecular weight of kraft lignin was developed. It was found that the fluorescence effect is the main source of error in the determination of the molecular weight of lignin, and the anisotropy and absorption strongly affect the second virial coefficient. It was also found that the anisotropic effect of kraft lignin dissolved in DMF and pyridine is contributed mainly by the anisotropic solvents rather than by the lignin particles and that the lignin solution in 0.1N NaOH is essentially optically isotropic. By using the correction procedure provided, the corrected molecular weight of lignin determined in three kinds of solvents varies only about ±10%, whereas the apparent weight-averaged molecular weight of the same lignin differs by ±57%. © 1993 John Wiley & Sons, Inc.