Hydrothermal liquefaction of biomass model compounds (cellulose, xylan, and lignin) was carried out between 350 and 400 degrees C using calcium formate (Ca(HCOO)(2)) as the in situ hydrogen source. In this study, high temperatures have been adopted for liquefaction as calcium formate acts as a hydrogen donor in that range. Ca(HCOO)(2) catalyzes lignin degradation toward liquefaction and, consequently, increases the biocrude yields by about 80%. Conversely, cellulose biocrude yields decreased when Ca(HCOO)(2) was introduced due to pronounced gasification. It was also observed that Ca(HCOO)(2) did not have any effect on xylan biocrude yield which remained almost the same. The maximum biocrude yields of cellulose, xylan, and lignin in the presence of Ca(HCOO)(2) were 7, 13, and 32 wt %, respectively. The excess hydrogen from Ca(HCOO)(2) was successful in upgrading the biocrude by removing oxygen which was confirmed by a significant decrease in atomic O/C ratio and increase in heating values (by 34.6%). The formate salt slightly upgraded the quality of cellulose and xylan biocrude through an increase in H/C ratio which in turn improved the heating values by 15.6 and 8.8%, respectively. The gas chromatography mass spectroscopy chromatograms of biocrudes with and without the hydrogen donor were compared, and reaction mechanisms were postulated to better explain the deoxygenation of biocrude by Ca(HCOO)(2). With excess hydrogen, the biocrude from lignin was rich in alkylated phenols, whereas aromatics were the major components in cellulose biocrude. The formate salt resulted in the formation of alkylated phenols and cyclic ketones in xylan biocrude via aldol condensation reactions.