The air oxidation of three Co-Ta alloys containing 15, 30, and 45 wt.%Ta was investigated with a thermobalance at 800°C. The oxidation morphologies and compounds were analyzed by scanning electrón microscopy, energy dispersive X-ray spectrometry, and x-ray diffraction. For the first two compositions, the oxidation kinetics follows the parabolic rato law while the Co-45Ta alloy presents a behavior intermedíate between linear and parabolic. The oxidation morphologies of Co-15Ta and Co- 30Ta alloys are very similar. They display an external Co3O4/CoO duplex scale and an internal heterophasic region (HR), in which both metallic phases were oxidized. The Co-45Ta alloy displays an external monolayer of Co2O4, and an internal oxidation zone (IOZ), where only the Ta-rich phase was oxidized.