During anaerobic digestion (AD) of lignocellulose- and protein-rich substrates known to contain a high load of aromatic compounds, various undesired intermediates can arise, which can accumulate and cause serious disturbances during the cascade-like AD process. The phenyl acids phenyl-acetic-(PAA), phenyl-propionic-(PPA), and phenyl-butyric acid (PBA) are such intermediates suspected to negatively affect the microbial community, resulting in a decreased biogas yield. In the present study, the impact of PAA, PPA, and PBA on the metabolism of CO