Hydration of borosilicate and soda-lime glass samples was performed to examine their stability in ambient atmosphere with a high humidity and at elevated temperatures. Hydration at 88 degrees C for two months resulted in a remarkable degradation in the transmittance of the soda-lime glass sample. The borosilicate glass demonstrated an insignificant decrease in its transmittance. We have concluded that the soda-lime glass substrates cannot be used as transparent superstrates for thin-film solar cells.