Abstract This study presents an in depth investigation of the coke chemistry occurring in FCC and USY catalysts during co-processing of fossil feeds (VGO type) blended with various types of upgraded bio-oils produced by fast pyrolysis of lignocellulosic bio-mass. It includes a brief survey of previous studies devoted to the effect of co-processing on FCC products yield and quality. A combination of two main processes is proposed to account for the marked increase in coke formation in the presence of oxygenated molecules in the reacting feed: (i) the conventional cracking route for the VGO fossil hydrocarbons leading to essentially graphitic coke deposited preferentially in the USY zeolite micropores, and (ii) the conversion of lignin fragments into hydrocarbons, residual light oxygenates (essentially phenolic type) and finally “bio-coke” which accumulates in mesopores as less structured coke as compared to the harder coke issued from hydrocarbons condensation. These two routes which are monitored by the catalysts structure, texture and acidity are strongly interacting via hydrogen transfer between light hydrocarbons and phenolic type fragments. A tentative mechanistic scheme is proposed.