The capacitance of the Na2O–SiO2 glass was measured in situ during heat treatment at various frequencies, 20, 100, 1, 3, 10, and 30 kHz. It was found that the capacitance of the glass abruptly decreases after a certain duration. The glass was quenched at this stage. It was confirmed by the X-ray diffraction and transmission electron microscopy that this decrease of capacitance was associated with the formation of crystallites in the glass matrix. The size of crystallites was observed to be in the range of about 10 nm.