Bio-oils (BOs) obtained from sweet sorghum stalk (SSS) methanolysis (BOm) and ethanolysis (BOe) at 300 degrees C for 30 min were analyzed with an electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer (ESI FT-ICR MS). In negative-ion mode, the predominant compounds in BOm and BOe are On (n = 1-10) class species with double bond equivalent (DBE) values of 1-14 and carbon atom numbers (CANs) of 5-35. The most abundant class species in BOm and BOe are O-4 and O-3, respectively. According to the distribution of On class species and the van Krevelen diagram, lignin-derived compounds are the main acidic species in the BOs. In addition, N1On (n = 0-10) class species with DBE values of 1-14 and CANs of 7-35 were also identified. In positive-ion mode, most of detected compounds in BOm and BOe are N2On (n = 1-6) class species with DBE values of 1-19 and CANs of 10-40, in which N2O2-N2O4 class species with DBE = 8-12 and CAN = 19-28 are the most abundant. These results facilitate understanding thermochemical behavior of SSS. (C) 2015 Elsevier Ltd. All rights reserved.