A method for depositing at least two layers of a metal-supported intermediate temperature solid oxide fuel cell (IT-SOFC) electrolyte, comprising depositing at least two layers of metal oxide crystalline ceramic upon a surface of a substrate, the method comprising the steps of: (i) depositing a solution of a soluble salt precursor of a metal oxide crystalline ceramic onto said surface of said substrate to define a layer of said solution of said soluble salt precursor on said surface; (ii) drying said solution of said soluble salt precursor to define a layer of said soluble salt precursor on said surface; (iii) heating said soluble salt precursor on said surface to a temperature of between 150 and 600 °C to decompose it and form a layer of metal oxide film on said surface; (iv) repeating steps (i)-(iii) at least one additional time, said solution of said soluble salt precursor being deposited onto said metal oxide film, such that said metal oxide film on said surface comprises a plurality of layers of metal oxide; (v) firing said substrate with said metal oxide film on said surface at a temperature of 500-1100 °C to crystallise said metal oxide film into a layer of metal oxide crystalline ceramic bonded to said surface of said substrate; and (vi) repeating at least once steps (i)-(v), wherein the surface is the layer of metal oxide crystalline ceramic previously produced in step (v), wherein each of steps (ii), (iii) and (v) is performed in an air atmosphere, wherein: in said first set of steps (i)-(v) said surface of said substrate is generally impermeable to said soluble salt precursor and is a mixed ionic electronic conducting electrolyte ceramic material and said metal oxide crystalline ceramic is ion-permeable, electrically insulating, and gas-impermeable; and in a final set of steps (i)-(v) said substrate is the layer of metal oxide crystalline ceramic previously produced, and said metal oxide crystalline ceramic which is produced is a mixed ionic electronic conducting electrolyte material. (i) depositing a solution of a soluble salt precursor of a metal oxide crystalline ceramic onto said surface of said substrate to define a layer of said solution of said soluble salt precursor on said surface; (ii) drying said solution of said soluble salt precursor to define a layer of said soluble salt precursor on said surface; (iii) heating said soluble salt precursor on said surface to a temperature of between 150 and 600 °C to decompose it and form a layer of metal oxide film on said surface; (iv) repeating steps (i)-(iii) at least one additional time, said solution of said soluble salt precursor being deposited onto said metal oxide film, such that said metal oxide film on said surface comprises a plurality of layers of metal oxide; (v) firing said substrate with said metal oxide film on said surface at a temperature of 500-1100 °C to crystallise said metal oxide film into a layer of metal oxide crystalline ceramic bonded to said surface of said substrate; and (vi) repeating at least once steps (i)-(v), wherein the surface is the layer of metal oxide crystalline ceramic previously produced in step (v), in said first set of steps (i)-(v) said surface of said substrate is generally impermeable to said soluble salt precursor and is a mixed ionic electronic conducting electrolyte ceramic material and said metal oxide crystalline ceramic is ion-permeable, electrically insulating, and gas-impermeable; and in a final set of steps (i)-(v) said substrate is the layer of metal oxide crystalline ceramic previously produced, and said metal oxide crystalline ceramic which is produced is a mixed ionic electronic conducting electrolyte material.