The effect of heat treatment on the Fourier transform infrared spectrometry (FTIR) reflection peak position of the Si-O structural band near 1087 cm(-1) of a liquid-crystal-display substrate glass was investigated. The FTIR peak position shifted more slowly with heat-treatment time, and to a greater extent than expected, for the fictive temperature change. The isochronal shift of the FTIR peak as a function of the heat-treatment temperature exhibited a good correlation with the change of chemical durability in an HCl solution, suggesting that the infrared peak shift was the result of the phase separation induced by the heat treatment. The separated phase, with spherical particles similar to 50 nm in diameter, was observed by scanning electron microscopy in a glass sample heat-treated at 750 degreesC for 2500 h. The FTIR peak shift measurement appears to be a sensitive method for detecting the early stage of the glass-in-glass phase separation for borosilicate glasses.