We investigated the effects of temperature (250-350 degrees C), reaction time (5-31 min), and solid loading (5-25 wt %) on hydrothermal liquefaction of Spirulina, Miscanthus, and primary sewage sludge using a central composite study design. Response surface methodology was used to identify maxima/minima for yields of gas, bio-crude, aqueous phase (AqP), and solid residue (SR), while the coefficients were used to identify the flux of the four product fractions. Effects on carbon recovery and contents of nitrogen and oxygen of the bio-crude were also assessed. The high bio-crude yields of Miscanthus and sewage sludge mainly resulted from low yields of SR Higher solid loading of Spirulina and Miscanthus increased bio-crude yields, while carbon recovery often improved with higher temperature and longer reaction time. Common to all feedstocks, the amount of degraded biomass displaced to the AqP decreased with increasing solid loading but simultaneously resulted in more nitrogen in the bio-crude.