Compositionally graded mullite/ZrO2 coatings, have been tested as environmental barrier coatings (EBCs) for protection against water vapor corrosion of Si-based ceramic components intended for application in turbine engines. Four and five layered systems were engineered by plasma spraying over SiC substrates consisting of a Si bond coat layer, 2 or 3 mullite/ZrO2 composite graded layers as middle layers and a nanostructured YSZ topcoat. These coatings were heat treated at 1300 °C in both stationary and thermal cycling conditions in a controlled water vapor environment. The effect of these ageing conditions on the coatings was comparatively investigated. Crystallization of the composite coatings and sintering of the YSZ topcoat was perceived. A reduction of SiO2 content was detected in the composite layers before aging. The porosity of the coating did not change appreciably with the ageing tests and only the evolution of the pre-existing cracks and the growing of a thermally grown oxide layer can be highlighted as the major effect of the ageing tests.