To shed light into the influences of the shape of the main hydrocarbon families constituting FCC feedstocks (naphthenes and alkanes) and of the phenolic molecules size on the MFI additive function during bio-oils and traditional Fluid Catalytic Cracking feedstocks co-processing, the transformations of n-heptane in presence of phenol and guaiacol were performed over an HMFI zeolite, at 350 and 450 degrees C. For phenol, the results were compared with those previously obtained with methylcyclohexane. A lower impact of phenol was noticed for n-heptane than for methylcyclohexane due to the easier linear alkanes diffusion into the zeolite channels, which reveals that the phenolic molecules influence on the MFI additive function could not be as critical as envisaged for a purely naphthenic feedstock. Whatever the reactant, higher temperatures do not overcome phenol deactivating effect because, once inside the zeolite structure, phenol polar molecules diffusion is very limited. An increase of the coke molecules ramification and aromaticity was observed for the n-heptane transformation in presence of phenol, instead of a reduction of the coke production from the reactant, as noticed for methylcyclohexane. Therefore, for n-heptane the further HMFI deactivation due to phenol mainly proceeds by pore blocking, contrarily to that verified for methylcyclohexane. Furthermore, the greater the size of the phenolic molecules present in the hydrotreated bio-oils, the lower their effect on the MFI additive action. In fact, bulky oxygenated molecules are preferential adsorbed close to the zeolite outer surface, from which they are more easily removed at higher temperatures. This explains the lower guaiacol deactivating effect at 450 degrees C, when compared with phenol. (C) 2010 Elsevier Ltd. All rights reserved.