The corrosion of an ex-service 7-YSZ coated high-pressure turbine blade by Si-undersaturated, Fe-,Ti-rich CMAS (FT-CMAS) was investigated. The constituents of the FT-CMAS deposit and its synthetic laboratory counterpart are identified by means of analytical SEM and XRD. FT-CMAS mainly consists of rhonite-, melilite-, and clinopyroxene-type solid solutions. FT-CMAS reacts with the 7-YSZ thermal barrier coating to a continuous layer of a Ca,Zr-rich garnet phase also known as kimzeyite. The microstructural analysis indicates that kimzeyite has formed via solid-state reaction. The formation of kimzeyite was studied in laboratory using YSZ/FT-CMAS diffusion couples and powder compacts. Experiments provide strong evidence that kimzeyite is forming predominantly from YSZ and melilite, whereas clinopyroxene and especially rhonite appear relative stable versus YSZ. The preferred kimzeyite formation from melilite and YSZ is explained by the availability of sixfold coordinated lattice sites for Zr4+ on melilite decomposition and subsequent garnet crystallization.