Pyrolysis is a promising method for converting biomass to biofuels. However, some of pyrolysis oil's physiochemical properties still limit its commercial applications. In this study, the autohydrolysis pretreatment at 175 +/- 3 degrees C for 40 min was conducted to improve the resulting pine pyrolysis oil's properties as a fuel. During autohydrolysis, deacetylation and decomposition of hemicellulose was observed by ionexchange chromatography and Fourier transform infrared spectroscopy (FT-IR). In addition, the cleavage of lignin ether bonds was clearly determined by C-13 cross-polarization/magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR). Phosphitylation followed by P-31 NMR analysis of the heavy oils gave detailed structural information of the hydroxyl groups; the results revealed that autohydrolysis pretreatment led to a reduction of carboxyl acids in the heavy oils generated at all three pyrolysis temperatures ( 400, 500, and 600 degrees C). The P-31 NMR analysis also revealed that autohydrolysis pretreatment led to a reduction of condensed phenolic hydroxyl groups in the heavy oils produced at 600 degrees C. H-1-C-13 heteronuclear single-quantum correlation (HSQC) NMR analysis showed that at a pyrolysis temperature of 600 degrees C, the pretreated pine produced lower methoxy group constituents. Both P-31 and HSQC NMR results indicated that autohydrolysis pretreatment increased levoglucosan yields in the bio-oils. (C) 2017 Elsevier Ltd. All rights reserved.