The effects of composition and metal particle size of platinum catalysts on ethane dehydrogenation were investigated on Pt x Sn 100− x /Mg(Al)O (70⩽x⩽100) catalysts prepared with average particle sizes between ∼2 and 7nm. At high conversions, catalyst deactivation from coke formation was a strong function of particle size and Sn/Pt. Deactivation decreased significantly with decreasing particle size and increasing Sn addition. To understand the true initial activity of un-deactivated catalysts, further experiments were run at low residence time conditions to limit ethene formation. For a fixed average particle size, ethane TOF and the selectivity to ethene increased with increasing content of Sn in the PtSn particle. For Pt and Pt 3 Sn compositions, ethane TOF increased with increasing particle size, while the selectivity to ethene was not strongly affected. The observed effects of particle size and composition are attributed to a combination of geometric and electronic factors.