High energy surface x-ray diffraction (HESXRD), x-ray reflectivity (XRR), mass spectrometry(MS) and surface optical reflectance (SOR) have been combined to simultaneously obtainsub-second information on the surface structure and morphology from a Pd(100) model catalystduringin situoxidation at elevated temperatures and pressures resulting in Pd bulk oxideformation. The results show a strong correlation between the HESXRD and SOR signalintensities during the experiment, enabling phase determination and a time-resolved thicknessestimation of the oxide by HESXRD, complemented by XRR measurements. The experimentsshow a remarkable sensitivity of the SOR to changes in the surface phase and morphology, inparticular to the initial stages of oxidation/reduction. The data imply that SOR can detect theformation of an ultrathin PdO surface oxide layer of only 2–3 Å thickness.