In this study, the mechanisms of anaerobic digestibility of cattail improved by microwave irradiation pretreatment were explored. Both Box-Behnken design and response surface methodology (RSM) were used to optimize the microwave irradiation pretreatment with regard to chemical oxygen demand equivalent of the products formed in the anaerobic digestion. The optimum products obtained was as follows: substrate loading of 17.0 g volatile solids/L, microwave intensity of 500 W, and irradiation time of 14.0 min. The product formation rate increased by 32% and product yield by 19% after microwave irradiation pretreatment at 100 degrees C, as compared with the conventional water-heating pretreatment under the same conditions. The X-ray diffraction (XRD) analysis showed that the crystallinity index of cattail was reduced by 9.4% with the microwave irradiation, compared with the conventional heating pretreatment. The atomic force microscope analysis further confirmed that the recalcitrant structure of wax and lignin covering on the cattail surface was disrupted by microwave irradiation. These results demonstrate that the breakdown of the crystalline and physical structure of cattail caused by microwave irradiation pretreatment improved its anaerobic digestibility. (c) 2011 Elsevier Ltd. All rights reserved.