Interlayer excitons (IXs) in transition metal dichalcogenide moiré heterobilayers offer a versatile platform for exploring strongly correlated quantum phases. Despite recent progress in probing excitonic properties, the thermalization dynamics of IXs, particularly within moiré superlattices, remain poorly understood due to their intrinsic weak oscillator strength. Here, we investigate how IXs are thermalized in both R- and H-stacked ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ heterobilayers using a moiré-resolved ultrafast spectroscopy that probes intralayer exciton states with distinct atomic registry and spatial localization. We resolve the formation and cooling kinetics of IXs, revealing that thermalization proceeds via phonon-mediated scattering on a timescale of $\sim 15\mathrm{ps}$. Our results highlight the role of exciton localization and site-specific interactions in governing IX dynamics and establish a spatially sensitive approach to exciton spectroscopy beyond the conventional momentum-space framework.

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• Electron relaxation
• Excitons
• Localization
• Transition metal dichalcogenides
• Twisted heterostructures
• Transient absorption spectroscopy
• Ultrafast femtosecond pump probe