The addition of Mg2+ modified zeolite at 0.1 g g(-1) of volatile suspended solids (VSS) increased the potential specific methanogenic activity (PSMA) and the apparent kinetic constant, k(0), showing PSMA and k(0) values 15 and 2 times higher than those observed for control reactors during the methanogenesis of acetate and methanol, respectively. In addition, the population of hydrolytic, acetogenic and methanogenic bacteria increased 100, 10 and 100 times compared to that observed in the controls. The anaerobic degradation of methanol and acetate occurred in one and two stages, respectively.