In a two-part study, the depletion process of an industrially applied NiCoCrAlYRe/IN738LC overlay coating system was both simulated and experimentally validated. This first part focuses on the experimental evaluation of the system during isothermal long-term exposure at 900 °C and 1000 °C for up to 7000 h. Phase identification was carried out using SEM, EDS, EBSD and HT-XRD, complemented by CALPHAD-based thermodynamic calculations. The objective was to establish a comprehensive understanding of the microstructure and the diffusion-driven degradation mechanisms of the coating system at both investigated temperatures. These findings form the foundation for the experimental long-term validation of the thermodynamic-kinetic diffusion simulations presented in the second part of this study. At both temperatures γ-(Ni) and β-NiAl were identified as the main MCrAlY phases. On free-standing coating a minor content of Cr-rich precipitates was associated to a mix of α-Cr and σ-(Co,Cr) at 900 °C and solely α-Cr at 1000 °C. On overlay coating system those precipitates did transform into Cr23C6 during the heat treatment starting from the substrate/coating interface. At 1000 °C, after 1500 h, homogenous precipitation of a second type of γ′-Ni3Al was ascertained in the γ-(Ni) grains across the entire coating. Their nano-scaled, blocky structure was similar to that found in Ni-base alloys. Rapid quenching experiments indicated that they stem from the cooling process.