The advent of ''biological pulping'' technologies necessitates a critical assessment of standard lignin determination techniques and their applicability to microbially degraded lignocelluloses. Oxidation-based lignin determination techniques have found worldwide acceptance by pulp-and-paper industries, as is reflected in the use of ''Chlorine'' and ''Kappa'' number standard techniques. The applicability of these techniques to biodegraded lignocelluloses, however, is limited by structural changes in the lignin due to enzymatic action. The reliability of the chlorine number method for lignin determinations during microbiological degradation was tested and it was found that chlorine numbers of decayed lignin sulfonates were 10-15% lower than those of the undecayed preparations. This article reports on the applicability of a permanent oxidation-based lignin determination method (Kappa-number method) to biodegraded lignin preparations.