Co2MnSi films were recently reported to show a significant increase in current-perpendicular-to-plane-giant-magnetoresistance upon annealing. Here, nuclear magnetic resonance was used to study the impact of annealing on the structure of such films. Below 550¿°C, no long-range L2 1 -order is observed, while annealing above 550¿°C leads to the formation of the ideal L2 1 configuration, however, with a distinct degree of off-stoichiometry. Further evidence from restoring field measurements hints that interdiffusion may account for the drop in magnetoresistance observed for samples annealed above 600¿°C. These results show that optimizing films for spintronics involves the identification of the best annealing temperature, high enough for long-range order to emerge, but low enough to maintain smooth interfaces.