The effect of chlorine addition to the feed stream was studied for the oxidative dehydrogenation of ethane over Pt/alpha-Al2O3 monoliths at short contact times. The addition of I part Cl-2 per 100 parts O-2 (similar to0.25% of the feed) resulted in increased ethane conversion and increased ethylene selectivity, and consequently higher ethylene yields. However, higher levels of chlorine addition (up to 5 Cl-2 per 100 O-2), while resulting in even higher ethane conversion, resulted in reduced selectivity to ethylene and no improvement in ethylene yields. The catalysts demonstrated enhanced performance even after the chlorine was removed from the feed; this was attributed to modification of the catalyst surface. The addition of chlorine resulted in a doubling of Pt dispersion, which persisted even after chlorine was no longer present on the surface. It is hypothesized that this change in catalyst microstructure is responsible for the observed improvements caused by chlorine addition.