Hot corrosion of an in-service EB-PVD coated 1st stage high-pressure turbine (HPT) blade following an interactive CaSO4 infiltration/CMAS deposition process has produced reactive interfaces with the yttria stabilized zirconia (YSZ) top coat and the TGO layer which were investigated by analytical scanning and transmission electron microscopy (TEM). Despite complete CaSO4 infiltration of the coating macroscopic damage of the airfoil was confined to the pressure surface close to the trailing edge. Since the bulk composition of the CaSO4/CMAS deposits exhibited a high lime-to-silica ratio CaSO4/CMAS/YSZ interfaces typically involved a Ca-Zr-rich layer followed by fine-grained Ca-FSZ particles enveloping rounded YSZ column tips. No interfacial reaction was observed between the YSZ column walls and the CaSO4 pore fillings. Possible terms for the formation of this unique CaSO4/CMAS microstructure are evaluated. Zirconia destabilization upon hot corrosion is discussed in the light of phase compatibilities in the systems CaO-Y2O3-ZrO2 and CaO-ZrO2-SiO2 and put into perspective with previous work on both, natural and synthetic hot corrosion deposits.