Coagulant addition and improved mixing conditions have been used in anaerobic membrane bioreactors (AnMBR) to improve membrane performance. Before coagulant was added, a flux of 8 L/m2 h was applicable and transmembrane pressure (TMP) increased from 1 kPa to 10 kPa in 5 days. However, after the coagulant was added, a flux as high as 50 L/m2 h was achieved with no noticeable increase in TMP during six hours of operation. Furthermore, at the same high flux, a long-term experiment showed that TMP increased to approximately 3 kPa in 20 days. Apparently, the applied coagulant significantly improved membrane performance. The reduction in the number of small particles was identified as the main cause for the high flux. However, the number of submicon particles increased in the long-term experiment. In addition, a model was developed that adequately described the TMP development in the short-term and long-term experiments. According to this model, the deterioration in specific cake resistance resulted in a sharp TMP increase in the long-term experiment. In addition, experiments showed that the effect of coagulant on sludge activity was minimal. This study demonstrated that the applied coagulant and reactor operation conditions (mixing properties) have potentials of interest for improving the membrane flux in AnMBR.