The interaction at the metal–metal interface of the Al–Si–Cu/MoSi2bilayer system has been investigated as a function of the Cu concentration. The Al in the Al–Si–Cu alloy reacts with the MoSi2at temperatures above 400 °C. Al atoms diffuse into the MoSi2and the excess Si diffuses into the Al layer. After the reaction, the amorphous MoSi2becomes crystalline Mo(Al,Si)2. The excess Si in the MoSi2layer forms precipitates in the Al layer. The Cu concentration in the Al alloy affects the reaction rate with 1% Cu having the largest effect followed by 0.5% Cu and then 3% Cu. A model is presented that explains this phenomenon. The Al–Si alloy without Cu does not react with MoSi2below 450 °C.