The effects of exposure conditions on the microstructural changes of the oxynitride Y-Al-Si-O-N glass system were investigated. The oxynitride glass was exposed to dry O-2 gas, or water vapor containing either O-2 or Ar, at temperatures between 1133 and 1183 K. A porous scale layer was formed by exposure to water vapor, while a non-porous deteriorated scale was obtained only by exposure to dry O-2. Formation of the porous layer was promoted by the presence of oxygen in the water vapor. The corrosion rate of the oxynitride glass in humidified O-2 near the glass transition temperature followed a linear rate law. The morphology of the porous layer was strongly dependent on the exposure temperature, which may be due to the significant decrease in the viscosity of the glass with increasing temperature. The permeability of the porous layer cut from the exposed glass behaved according to Darcy's law; therefore, this layer was considered to be composed of three-dimensional continuous pores. The microstructure of the porous layer could be controlled by the exposure temperature, so that a graded porous glass with different morphological characteristics within the layer could be obtained by exposure to humidified gases at different temperatures.