Abstract We followed the first stages of liquid–liquid phase separation in an alkali-borosilicate glass by complex impedance spectroscopy at constant temperatures below and above the glass transition temperature, T g. We found a new feature of the electrical conductivity behavior: at temperatures below T g, the electrical conductivity, σ, diminished with time, while above T g, the electrical conductivity increased. The activation energy, E a, for electrical conductivity was largest for samples heat-treated below T g. Other techniques, such as simultaneous small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), and transmission electron microscopy (TEM) were also used. After annealing the glass inside the immiscibility gap, TEM shows a structure with a few Angstroms in size. Above and below the glass transition temperature, in situ SAXS isothermal experiments show an increase of the scattered intensity with time, while WAXS confirms that no crystallization occurred. These facts suggest that glass heat treated below T g develops a SiO 2 rich matrix with lower electrical conductivity and isolated sodium- rich regions. Annealing above T g (of the quenched glass) seems to produce an interconnected Na-rich phase, which dominates the electrical conductivity.