Co-refining of biomass derived pyrolysis liquids (PL) in a Fluid Catalytic Cracking (FCC) unit with crude oil fractions is a promising route to produce second generation biofuels. To improve the yield and ensure co-processed fuel quality, a hydrotreating step can be deployed prior to co-processing. In this paper the conversion, yields and naphtha or gasoline composition are analyzed during a 10 wt.% PL/90 wt.% vacuum gasoil (VGO) co-processing step as a function of the PL hydrotreating severity, expressed in terms of hydrogen consumption. The naphtha yield was not impacted by the hydrotreatment procedure and always slightly higher than that of pure VGO cracking. An optimum in the naphtha quality in terms of octane rating was found as a function of the PL upgrading severity. The optimum was achieved with mildly hydrotreated PL's, corresponding to a H-2 consumption of 202 NL/kg(PL). These mildly hydrotreated PL's led to similar coke formation during co-refining as pure VGO cracking. These results indicate that the upgrading of PL's should be restricted to a hydrogen consumption of less than 200 NL/kg(PL). (C) 2016 Elsevier Ltd. All rights reserved.