Noble metal-based catalysts are ubiquitous because of their high activity and stability. However, they irreversibly deteriorate over time especially in high-temperature applications. In these conditions, sintering is the main reason for deactivation, and understanding how sintering occurs gives the opportunity to mitigate these detrimental processes. Previous studies successfully distinguished between two fundamental sintering modes, namely, particle migration and coalescence (PMC) and Ostwald ripening (OR). However, differentiation between surface- and vapor-mediated Ostwald ripening processes has not been demonstrated yet, even though it is crucial information to tune metal/support interactions and stabilize catalysts. Here, we demonstrate that surface- and vapor-mediated ripening occur in two distinct regimes of temperature with some overlap using Pt and Pd catalysts prepared from colloidal nanocrystals as precursors. By either co-impregnating the two metal nanocrystals on the same grain of alumina support or by physically mixing powders of the two distinct metal catalysts, we tune the intermetal particle distance between nanometers and micrometers. We then use methane complete oxidation as a reporter reaction that occurs at higher rates on pure Pd and lower rates on alloyed Pd/Pt catalysts to trace the movement of Pt in the system. Aging the catalysts at different temperatures allows us to reveal that Pt initially sinters by surface-mediated ripening until ∼750 °C, but at temperatures above 800 °C, vapor-mediated ripening by PtO2 becomes the main sintering mechanism. This work demonstrates how colloidal catalysts allow unique insights into the working and deactivation mechanisms of supported systems.