We studied the reaction kinetics of sulfur oxidation on the Pd(100) surface by in situ high resolution x-ray photoelectron spectroscopy and ab initio density functional calculations. Isothermal oxidation experiments were performed between 400 and 500 K for small amounts (∼0.02 ML) of preadsorbed sulfur, with oxygen in large excess. The main stable reaction intermediate found on the surface is SO4, with SO2 and SO3 being only present in minor amounts. Density-functional calculations depict a reaction energy profile, which explains the sequential formation of SO2, SO3, and eventually SO4, also highlighting that the in-plane formation of SO from S and O adatoms is the rate limiting step. From the experiments we determined the activation energy of the rate limiting step to be 85 ± 6 kJ mol−1 by Arrhenius analysis, matching the calculated endothermicity of the SO formation.