The glycerol partial oxidation reaction over Pt/Al2O3 catalysts was studied under basic (NaOH/GLY molar ratio 4) and base‐free conditions (NaOH/GLY molar ratio 0). Catalysts with small (2.95 nm) and large particle sizes (260.83 nm) were synthesized according to the use of different reducing agents, formaldehyde or sodium borohydride, and hydrazine, respectively. These different Pt particle sizes lead to a dramatic change in terms of activity, irrespective of the applied conditions. The biggest particles (i.e., 260 nm) seem to generate overoxidation products leading to a decrease in the carbon balance (to ~80%) while the smallest particles exhibit the highest initial glycerol transformation rate (i.e., ~10,000 mol h−1 molPt−1 under basic conditions at 60°C and ~2000 mol h−1 molPt−1 in the absence of a base at 100°C). In terms of selectivities, the main products are different as a function of the initial reaction conditions. For base‐free conditions, the two main products are glyceraldehyde and glyceric acid with a sum of selectivities always larger than 80%. Under basic conditions, the major product is glyceric acid while no trace of glyceraldehyde is detected.