The tetragonal to monoclinic phase transformation resulting from the hydrothermal ageing of 4-20 mass % yttria-stabilized zirconia plasma-sprayed coatings formed from commercial powders has been studied with respect to the Y2O3 distribution inside each coating. The phase transformation was prevented when the nominal Y2O3 concentration was greater than 8 mass % with its point to point distribution ranging between 6.82 to 9.90 mass %. It is suggested that a close correlation exists between the durability of the tetragonal phase in a humid atmosphere and the Y2O3 distribution in the tetragonal zirconia coatings. A content of 6.3-6.8 mass % Y2O3 was calculated to be necessary to prevent the transformation.