To gain insight regarding the orthophosphate influence on fatty acid dynamics and methane production during mesophilic anaerobic sludge digestion, batch experiments were conducted. The results showed that an orthophosphate concentration of 414 g P m(-3) accelerated acetotrophic methanogenesis, acetogenesis and acidogenesis. Lower or higher concentrations slowed these processes down. Three modifications of the Anaerobic Digestion Model No. 1 (ADM1) were compared by simultaneously fitting them to a multi-experiment (7 batches), multi-variable (6 measured variables) data set. A modified ADM1 model including Haldane kinetics of orthophosphate inhibition gave the best description of the measured data while the model using non-competitive acetate inhibition showed the poorest results. It is notable that the multi-experiment, multi-variable data set could be fitted with a single parameter set. The model with Haldane kinetics of orthophosphate inhibition together with the proposed parameters can be considered as a first attempt to describe the observed orthophosphate inhibition of anaerobic digestion. (C) 2014 Elsevier B.V. All rights reserved.