Emission control catalysts experience degradation under the severe operating conditions which can negatively impact the product distribution and conversion. Understanding and quantifying this impact is necessary to properly design the catalyst. This study elucidates the effect of hydrothermal aging on the performance of a Pt/gamma Al2O3 washcoated monolith used for oxidation of ammonia and of propylene, two reaction systems pertinent to the Ammonia Slip Catalyst (ASC) and Diesel Oxidation Catalyst (DOC), respectively. The Pt/gamma-Al2O3 catalyst was subjected to a feed stream containing H2O at 550 degrees C for over 250 h. The performance of the catalyst was measured at discrete intervals as it was progressively aged. The catalyst performance results reveal for ammonia oxidation that hydrothermal aging has a negligible impact on Pt/Al2O3 activity for temperatures below 250 degrees C but a significant detrimental impact for temperatures exceeding 300 degrees C. A (1D + 1D) reactor model was developed to predict the decline in the observed activity with aging time. Using kinetics and catalyst characterization data, the reactor model predicts that changes in washcoat morphology during aging is likely the underlying mechanism for the decline in activity of the catalyst. The results are corroborated with testing and modeling of propylene oxidation on the Pt/Al2O3 catalyst.