The challenge of enhancing the high-temperature oxidation resistance of TiAl alloys is hereby addressed by Co addition. Isothermal oxidation tests were conducted on the newly designed TiAl-Co alloys in laboratory air at 900 degrees C up to 100 h. Sintered microstructure, oxidation kinetics, scale structure, spallation resistance and oxidation mechanisms were systematically investigated. Results show that the original sintered microstructure of TiAl alloys mainly consists of matrix phases alpha(2)-Ti3Al/gamma-TiAl lamellae, while the Co addition leads to the formation of the two additional Co-rich phases of CoAl2Ti and Ti (Al, Co, Cr and Nb) at grain boundaries. The Co-doped TiAl alloys exhibit an improved high-temperature oxidation resistance compared with the Co-free alloy. The presence of the Co-rich phases network along the grain boundaries and Co-rich layer at the scale/substrate interface can hinder the inward diffusion of oxygen and the outward diffusion of Ti and Al, thereby suppressing the growth of oxide scale and improving the spallation resistance of TiAl alloys. As a result, the TiAl-3Co alloy possesses excellent oxidation resistance, with the minimum mass gain of 4.08 mg/cm(2), thinnest scale thickness of 17.8 mu m and without surface spallation or crack formation after isothermal oxidation for 100 h. This result would pave the way for designing high-temperature oxidation-resistant TiAl-based alloys.