A novel analysis method of the platinum L-3 and L-2 X-ray absorption edges of Pt/LTL catalysts indicates that chemisorbed hydrogen induces an antibonding resonance state just above the Fermi level at the Pt L-3 edge. The difference in energy of this antibonding state (E-res) with respect to the Fermi level is strongly influenced by the acidity/alkalinity of the LTL support: E-res decreases with increasing alkalinity. The decrease in E-res can also be correlated with the decrease of the neopentane hydrogenolysis turnover frequency of Pt/LTL with increasing support alkalinity. These results provide the most direct experimental evidence that the support acidity/alkalinity alters the actual chemical bond between the surface platinum atoms and the reacting molecule.