Within the context of the design of high-temperature brazing process for C and SiC-based composites, a basic study is presented here about the wetting and interfacial reactivity of Co-Ta alloys in contact with graphite or SiC. Wetting results are presented for the first time showing that Co-Ta alloys wet C or SiC fairly or excellently depending on the relative amount of Ta. The final interfacial microchemistry and microstructures are the result of the interplay between the typical interfacial phenomena of the pure elements. Specifically, depending on the alloy composition, dissolution of the ceramic phase by Co or formation of a continuous interfacial layer of TaC that prevents dissolution prevails. The discussion about the interfacial reactivity between liquid Co-Ta alloys and graphite or SiC, as well as the interpretation of solidification phenomena and the formation of interfacial phases, is supported by making reference to new multi-component phase diagrams calculated by CALPHAD method.