The pyrolysis behaviors of four types of pretreated switchgrass (torrefied at 230 and 270 degrees C, densification, and torrefaction at 270 degrees C followed by densification) were studied at three temperatures (500, 600, 700 degrees C) using a pyroprobe attached to a gas chromatogram mass spectroscopy (Py-GC/MS). The torrefaction of switchgrass improved its oxygen to carbon ratio and energy content. Contents of anhydrous sugars and phenols in pyrolysis products of torrefied switchgrass were higher than those in pyrolysis products of raw switchgrass. As the torrefaction temperature increased from 230 to 270 degrees C, the contents of anhydrous sugars and phenols in pyrolysis products increased whereas content of guaiacols decreased. High pyrolysis temperature (600 and 700 degrees C as compared to 500 degrees C) enhanced decomposition of lignin and anhydrous sugars, leading to increase in phenols, aromatics and furans. Densification enhanced depolymerization of cellulose and hemicellulose during pyrolysis. (C) 2014 Elsevier Ltd. All rights reserved.