Abstract The pyrolysis behaviors of four types of pretreated switchgrass (torrefied at 230 and 270 �C, densification, and torrefaction at 270 �C followed by densification) were studied at three temperatures (500, 600, 700 �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 �C, the contents of anhydrous sugars and phenols in pyrolysis products increased whereas content of guaiacols decreased. High pyrolysis temperature (600 and 700 �C as compared to 500 �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.