The alloys being used in high-temperature systems such as stationary gas turbines and aircraft engines are iron-, cobalt-and nickel-based superalloys, amongst which the latter is the most widely used for highest temperatures. However, the use of Ni-based alloys is limited to temperatures below 1,100°C. The experimental Co-Re-based alloys are promising for high-temperature applications for service temperatures beyond 1,200°C. The purpose of the present investigations, at this still early stage of the alloy development, is to gain a first insight into the oxidation mechanisms and to find ways to improve oxidation resistance of this class of materials. Thermogravimetric studies in combination with microstructural examinations of six model Co-Re alloys with different compositions showed the negative influence of rhenium on the oxidation resistance of Co-based alloys due to evaporation of rhenium oxide(s). Oxidation at 1,000°C in air yielded an oxide scale, that consists of a Co-oxide outer layer on a thick and porous Co-Cr oxide and a semicontinuous and therefore non-protective Cr-oxide film on the base metal substrate. This allowed for the vaporization of rhenium oxide formed during oxidation and hence led to a loss of Re. Computer-aided thermodynamic calculations were carried out to supplement the experimental analyses and were found to reasonably predict the stability ranges of the various oxide phases observed.