The treatment of biomass-derived fast pyrolysis vapors with solid acid catalysts (in particular HZSM-5 zeolite) improves the quality of liquid bio-oils. However, due to the highly reactive nature of the oxygenates, the catalysts deactivate rapidly due to coking. Within this study, the deactivation and product yields using steam-treated phosphorus-modified HZSM-5/γ-Al2O3 and bare γ-Al2O3 was studied with analytical Py-GC. While at a fixed catalyst temperature of 450 °C, a rapid breakthrough of oxygenates was observed with increased biomass feeding, this breakthrough was delayed and slower at higher catalyst temperatures (600 °C). Nevertheless, at all (constant) temperatures, there was a continuous decrease in the yield of oxygen-free hydrocarbons with increased biomass feeding. Raising the reaction temperature during the vapor treatment could successfully compensate for the loss in activity and allowed a more stable production of oxygen-free hydrocarbons. Since more biomass could be fed over the same amount of catalyst while maintaining good deoxygenation performance, this strategy reduces the frequency of regeneration in parallel fixed bed applications and provides a more stable product yield. The approach appears particularly interesting for catalysts that are robust under hydrothermal conditions and warrants further investigations at larger scales for the collection and analysis of liquid bio-oil.