show the yield of lignin in the filtrate (measured by using the Klason lignin method and the UV method) at different temperatures and pH values. As can be seen, there is a big difference between the results of the Klason lignin and the UV methods. The latter underestimated the concentration of the lignin in the filtrates, consequently, the calculated yield of lignin in precipitation was erroneously low. This is in agreement with earlier findings by Theliander[10]. The reason for this is probably because the recommended absorption constant for softwood black liquor, considered to be 24.6 (dm3/g
cm), was used in the calculation of the lignin concentration in the filtrates after precipitation. It is known that the absorption constant varies with composition, and in this Proceedings of the 16th ISWFPC case a lot of lignin was removed from the black liquor. Therefore it can be assumed that the UV method gave them erroneous results. Furthermore, considering that the black liquor sample investigated here was a softwood/hardwood mixture and that the absorption constant of hardwood black liquor is lower, 21 (dm3 /g cm), than the absorption constant of softwood, the calculated lignin yield based on this constant will be even lower. Therefore, the amount of lignin yielded is better determined using the Klason lignin method.