Appropriate evaluation of the process performance of biogas plants needs to consider the anaerobic biodegradability of the used biomass. Anaerobic biodegradability is limited by lignin, which is part in most substrates and, by extension, in digestates of biogas plants. Previous research has shown that the content of acid detergent lignin (ADL) in digestates can be predicted from measured gross calorific values (GCVs). The correlation of GCV of 34 digestate samples to an alternative measure for lignin, the Klason lignin (KL), is evaluated as well as the correlation of KL content and other chemical constituents to residual biomethane potential (BMP). Results indicate a very low correlation of chemical composition to GCV and BMP. A correlation of GCV to BMP was not observable. The results let conclude that evaluation of anaerobic biodegradability of digestates by measuring KL or predicting KL from GCV is not productive.