Lignocellulosic feedstock can be converted to bio-oil by direct liquefaction in a phenolic solvent such as guaiacol with an oil yield of >90 C% at 300-350 degrees C without the assistance of catalyst or reactive atmosphere. Despite good initial performance, the liquefaction was rapidly hindered by the formation of heavy components (molecular weight > 1000 Da), which increased the viscosity of the bio-oil upon recycling the bio-oil or a fraction of it as a liquefaction solvent. This paper explores the possibility to minimize the production of this undesirably heavy fraction by optimizing the process parameters such as temperature, heating rate, reaction time, and concentration of water. This study allowed us to find a compromise between maximizing the bio-oil yield and minimizing its heavy fraction. It also provides insight onto the reaction network of the liquefaction reaction, showing for instance that all product fractions, including the heaviest products and the char, are mainly direct liquefaction products rather than secondary reaction products, e.g. from bio-oil recondensation. However, the resulting heavy fraction is still too high to allow effective recycling of the bio-oil. Complementary approaches need to be investigated.