(Co,Ni)O solid solutions are considered as promising protective materials of O-2-evolving anodes for Al production. In this context, two solid-state synthesis methods, namely high-energy ball milling (HEBM) and calcination, have been evaluated for the synthesis of (Co,Ni)O solid solutions. In all cases, CoxNi1-xO solid solutions can be formed over the whole composition range. However, undesired WC contaminant is observed using the HEBM method due to the erosion of the milling tools. Their thermal stability in air has been analyzed by thermogravimetric analyzes (TGA) complemented by X-ray diffraction (XRD) analyses. It is shown that CoxNi1-xO solid solutions are stable at 1000 degrees C over the whole composition range whereas they are only stable for x <= 46 and x <= 22 at 800 degrees C and 700 degrees C, respectively. For higher Co contents, the formation of Co3O4 is observed. This is a relevant information for their future use for Al production, which can be done at different temperatures (similar to 700-1000 degrees C) depending of the electrolyte composition.