There is a need for replacement of fuel, especially transportation fuel, obtained from fossil resources by bio-based alternative. The thermochemical conversion is a feasible method to degrade and partially deoxygenate the natural resource lignocellulose to get bio-oils as fuel precursors. The present paper describes the liquid phase pyrolysis of wheat straw and poplar under inert atmosphere in n-hexadecane solvent. An autoclave batch reactor system was used. The influence of reaction temperature, residence time, solvent/biomass ratio and the effect of solid acid and base catalysts on the product distribution and yields were investigated. The reaction products were analyzed by means of CHNO elemental analysis, Solid Phase Micro Extraction-Gas Chromatograpy-Mass Spectroscopy (SPME-GC–MS), 1H and 13C-Nuclear Magnetic Resonance (NMR) spectroscopies. About 40–49 wt% of lignocellulosic carbon was successfully transferred to hexadecane. The best carbon yields were achieved at 350 °C. The main extracted products were alkyl and alkoxy derivatives of phenol and furan derivatives. By comparing seven parallel experiments in presence and absence of the catalysts no or only negligible catalytic effect was observed. The effect of heterogeneous catalysts was manifested only in splitting the primarily extracted macromolecules. The carbon/oxygen mass balances suggest that the extracted compounds have relatively high residual oxygen content.