The conditions and mechanism of bonding a tungsten sheet directly onto a Si80Ge20alloy by hot pressing have been studied. It was found that W/Si80Ge20bonds are formed only at relatively high temperatures. The pressure and time required for bonding decrease with increasing temperature. In the initial stage, the W/Si80Ge20reaction follows a parabolic kinetics, suggesting it is a diffusion‐controlled process. The activation energy is 4.9 eV, and the parabolic rate constant is given byKp=(4.2±0.1)×1018 exp[−(5.7±0.1)×104/T] (&mgr;m2/ min) A bonding mechanism is proposed based on scanning electron microscopic observations and chemical analyses of the interface. Selective reaction between silicon and tungsten leaves a Ge‐rich layer at the W/Si80Ge20interface, thus reducing the melting temperature. When the silicon content in the interfacial layer becomes sufficiently low, the layer melts at the hot‐press temperature. Molten materials fill in voids and microcracks by the capillary effect, resulting in an intimate contact and a strong bond. A mathematical model of Si diffusion is proposed. Solving the equation by the finite‐difference method, silicon composition profiles were obtained and the silicon diffusion coefficient was estimated.