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The contact angle for iron on a single-crystal alumina substrate varied from 40 to 120 deg by changing the gas atmosphere and droplet size. At constant droplet size, the contact angle was a strong function of the partial pressure of oxygen and increased as oxygen partial pressure decreased. However, at oxygen partial pressures less than 10 Ϫ18 atm, the contact angle markedly decreased as a zone of reactive wetting was reached due to interaction between alumina and the droplet. The effect of droplet size on the contact angle between liquid iron and a single-crystal alumina substrate was measured at constant oxygen activity and temperature. In an argon-hydrogen atmosphere where oxygen activity was constant, the contact angle increased with increasing droplet size and varied from 40 to 80 deg as the droplet radius increased from 1.5 to 4 mm. A similar though smaller effect was measured in CO atmospheres that were in equilibrium with carbon. In this case, the contact angle ranged from 105 to 118 deg as droplet size increased. Experiments in CO/CO 2 atmospheres showed little effect of droplet size at oxygen partial pressures less than 5 ϫ 10 Ϫ12 atm, but indicated a significant effect at oxygen partial pressures greater than 10 Ϫ11 atm.