No / The transient liquid phase bonding process has long been dealt with as a pure diffusion process at the joint interface, that is, as a mass phenomenon. In spite of the advances in the application of this technique to bond complex engineering alloys, the available models have failed to incorporate the effect of surface phenomena on the joining process. In this work, a new reaction–controlled diffusion formulation model is proposed, and the observation of experimental results of joining Al6061 alloy using thin single (50, 100 micron) and double Cu foils is recorded. This work directly unveils the unique role played by surface reaction–controlled diffusion rather than purely mass diffusion bonding process. Our experimental and modeling results reveal a conceptually new understanding that may well explain the joint formation in TLP bonding process.