Gallium-based low melting temperature alloys have been proposed as candidates for next generation thermal interface materials (TIMs) due to their high thermal conductivity (~30 W/m*K) and liquidity. However, poor wettability as well as embrittling and corroding effect of Ga on metals have limited their use by the electronics industry. Studies on the relationship between the evolution of thermal properties and interfacial reactions between Ga-based TIMs and metal substrates are thus vital for creating a path forward. We measured thermal conductivity and thermal interface resistance of eutectic Ga-In alloy (EGaIn) sandwiched between two Ni-plated Cu substrates following simulated assembly and accelerated aging. The rapid interfacial reaction between EGaIn and both Ni and Cu at elevated temperatures led to an increase in the thermal conductivity. Further study showed the change in thermal properties was due to the depletion of Ga in the system through intermetallic formation, creating a higher conductivity In-rich alloy.