;BACKGROUND: Lignocellulosic wastes such as herbal-extraction process residues (HPR) are not easily utilized by microorganisms owing to their physical shielding of cellulose imparted by the non-digestible lignin. Therefore, there is a great interest to develop an efficient pretreatment technique to disrupt recalcitrant structures of lignocellulosic wastes and improve renewable energy production. RESULTS: A microwave-assisted alkaline pretreatment (MAP) method has been developed for the enhancement of biogas production from HPR. The maximum cumulative biogas production reached 1477 mL when the HPR was hydrolyzed by MAP for 30 min with an alkali loading of 0.12 g-NaOH/g-HPR, where the maximum weight loss of HPR after biogas fermentation reached 72.1%. CONCLUSION: The present work demonstrated that MAP is a promising method for improving bioconversion of lignocellulosic wastes to biogas. HPR pretreated by combining microwave irradiation and alkali resulted in releasing more soluble substances from HPR and increasing the accessibility of HPR for anaerobic biodigestion compared with traditional alkaline pretreatment (AP) and microwave-assisted water pretreatment (MWP). (C) 2009 Society of Chemical industry