An in-service high-pressure turbine blade with a columnar, Y2O3-stabilized ZrO2 (YSZ) thermal barrier coating (TBC) fabricated by electron-beam physical vapor deposition was investigated to access the TBC hot corrosion mechanisms during turbine operation. The TBC exhibits a through-thickness pore filling with anhydrite-type CaSO4. Chemical analysis of the CMAS-type particle deposits reveals relatively low SiO2 but high CaO contents and substantial amounts of Fe2O3 and TiO2. The hot corrosion scenario observed at the YSZ column tips involves newly formed CaZrO3 and the garnet-type phase Ca3(Zr,Mg,Ti)2(Fe,Al,Si)3O12, also known as the mineral kimzeyite. The phase relationships were confirmed in laboratory experiments. CaSO4 as well as the particle deposits prove to be effective solvents for YSZ introducing distinct solid-state reactions. The results support the idea of a dual YSZ hot corrosion process. A first stage controlled by a SiO2-free Ca-source, most likely primary CaSO4 produces a thin CaZrO3 layer. A second, CMAS-type stage providing high concentrations of Fe2O3, TiO2, and SiO2 favors the formation of kimzeyite. The melting temperature of kimzeyite presumably defines a thermal operation limit for YSZ-based TBCs.