Metal nanoparticle (NP) sintering is a prime cause of catalyst degradation, limiting its economic lifetime and viability. During sintering, the average NP size increases but strong debate exists on which mechanism causes sintering, being either (i) NP migration and coalescence or (ii) Ostwald ripening by atomic transport between immobile NPs. Herein, the aim is to extract first-of-its-kind insights on the role of NP migration during NP sintering. By applying GIXPCS-GISAXS under true working conditions of the catalyst, the NP migration rate will be monitored in conjunction to its size evolution under different reaction conditions, allowing to unravel which mechanism governs NP sintering. This approach has never been applied in NP catalysis, and will unambiguously answer long-standing, foundational questions on NP sintering.