Transition metal carbides (TMCs) have attracted great interest due to their mechanical and catalytic properties but their syntheses generally require energy-consuming processes with temperatures above 800 °C. We report here a solid-state metathesis reaction between metal chlorides (ZrCl 4 , NbCl 5 , MoCl 3 , MoCl 5 , HfCl 4 , TaCl 5 , WCl 6) and potassium dispersed in carbon (graphite or acetylene black). The reaction is initiated at room temperature or with gentle heating below 75 °C. Within seconds, it produces carbon-supported carbide or metallic nanoparticles of diameter below 50 nm. The phase speciation depends on the metal and different process parameters. In some cases (ZrCl 4 , NbCl 5 , HfCl 4 , TaCl 5), the metal is further converted into a hydride upon neutralization of the reaction crude with ethanol. The products were characterized by powder X-ray diffraction (PXRD) and Transmission Electron Microscopy (TEM). Based on thermodynamic and kinetic considerations, we propose a mechanism explaining the coexistence of several phases (metal, carbide, hydride) and their occurrence at each step of the reaction.