Bifunctional catalysts with activity for both hydrogenation and dehydration have been frequently investigated for hydrodeoxygenation (HDO). Here, we report the application of organophosphonic acids (PAs) to Pd/Al2O3 catalysts for low-temperature vanillin HDO. Reaction studies indicated that PA-modification significantly improved the liquid-phase HDO activity; the yield to the desirable product, p-creosol (CR), increased from 2.5% to 87% at 50 degrees C. This improvement was attributed to the creation of metal/acid bifunctional sites upon PA modification. In addition, HDO activity positively correlated with the Bronsted acidity of the PA modifier, which could be tuned by adjusting the PA tail functionality.