In this work, we provide detailed information on the change in product distribution and bio-oil quality during extended feeding of biomass derived fast pyrolysis vapors over ZSM-5. The effect of catalyst deactivation by coking on the resulting oil product characteristics was clarified in order to determine when the vapor upgrading should be stopped and the regeneration initiated. Obtaining a stable catalytic fast pyrolysis (CFP) oil while maintaining good energy recovery is important within the context of potential coprocessing of these oils with petroleum feedstocks via fluid catalytic cracking (FCC) or hydrotreatment of the whole CFP oil. Wheat straw derived fast pyrolysis vapors were upgraded in an ex-situ fixed bed reactor containing a steamed ZSM-5 catalyst at 500 degrees C. Oils were collected both for runs starting the upgrading over a fresh (or regenerated) catalyst and for runs which were continued over an increasingly coked zeolite. The oils were characterized for water content, elemental analysis, total acid number (TAN), chemical composition by gas chromatography mass spectrometry with flame ionization detection (GC-MS/FID), size exclusion chromatography (SEC), evaporation characteristics by thermogravimetric analysis (TGA), H-1 nuclear magnetic resonance (NMR), C-13 NMR, and two-dimensional heteronuclear single-quantum correlation (2D HSQC) NMR. With increasing biomass-to-catalyst mass ratio (B:C), the yield of deoxygenated hydrocarbons decreased, accompanied by a breakthrough of primary pyrolysis vapors leading to an increasing organic liquid yield. The oxygen content of the condensed, phase separated oil fraction increased and the molar O/C ratio of 0.05 and TAN of 6 mg KOH/g for oil collected during B:C = 0-1.1 increased to O/C = 0.18 and TAN = 14 mg KOH/g for oil collected during B:C = 3.6-6.2. Oil produced at 90% reduced catalyst amount and B:C = 0-6.5 and 0-12.9 increased the carbon recovery into the oil product to 23% and 27%, respectively but led to an increase in O/C ratio from 0.18 to 0.22, thus approaching the noncatalytic reference case (SiC bed at 500 degrees C) of O/C = 0.24. Clear differences in the evaporation behavior of the collected oils were observed, with a shift to more volatile fractions and less charring for products obtained at low B:C ratio. Characterization of the upgraded oils with C-13 NMR and H-1 NMR indicated a clear enhancement of the aromatics content and a reduction of sugar and aldehyde compounds. The concentration of carbon within carbonyl, carbohydrates, and methoxy/hydroxyl groups was effectively reduced for oils obtained at low B:C ratios. Catalyst characterization was performed with X-ray fluorescence (XRF), ammonia temperature-programmed desorption (NH3-TPD), N-2 and Ar-physisorption, transmission electron microscopy (TEM), and X-ray diffraction (XRD). After steaming and four repeated upgrading/regeneration cycles corresponding to an accumulated B:C ratio of 40, the zeolite/s concentration of strong acid sites measured by NH3-TPD (T-des > 275 degrees C) reduced from 0.43 mmol/g for the calcined version to 0.07 mmol/g and the Brunauer-Emmett-Teller (BET) surface area decreased from 468 to 385 m(2)/g. The hot gas filter upstream of the zeolite bed was found effective in preventing accumulation of potassium on the catalyst.