Pretreatments for enzymatic saccharification are crucial for the establishment of lignocellulosic biorefineries. In this study, we focused on ammonium ions and peroxometal complexes as potential delignifying agents. We first examined the pretreatment of beech wood with nine different ammonium salts in the presence of H2O2. Significant pretreatment effects were found only for ammonium molybdate, which is transformed to a peroxometal complex on reacting with H2O2. Since microwave sensitizer effects are expected for (peroxo)molybdate. beech wood was pretreated using external heating and microwave irradiation. As a result, a maximum sugar yield of 59.5% was obtained by microwave irradiation at 140 degrees C for 30 min, while external heating in an autoclave gave a sugar yield of 41.8%. We also found that an ammonium ion is the key counterion accelerating the pretreatment with molybdate. These results highlight the powerful selective delignifying capability of the H2O2-activated ammonium molybdate system energised by microwave radiation. (C) 2010 Elsevier Ltd. All rights reserved.