Several Au/Al2O3 catalysts were prepared and evaluated in the oxidation of l-arabinose. Metal cluster size was varied by application of different preparation methods (DPU and DIE), washing agents (ammonia or water), calcination temperatures, and concentrations of the initial precursor solutions. The structure sensitivity of the reaction over these catalysts was proven. A volcano relationship of activity with the mean particle size was observed, the catalyst prepared by DPU being the most active one. The catalyst electric potential was correlated with the particle size by an inverse volcano shape. At 30% of conversion, the DPU sample exhibited the lowest potential.