Microstructures of sodium borosilicate glasses with intermediate silica content, doped with CuO, were examined. Electron micrographs of heat-treated glasses show two randomly distributed phases. DC conductivity measurements indicate that alkali oxide is associated with B₂O₃. CuO is assumed to act as a network modifier in the borate matrix and does not affect the phase separation process. Optical absorption in the UV and visible regions was also examined. Phase separation causes a shift of the UV edge towards longer wavelengths, which is considered to be a result of light scattering on SiO₂-rich particles produced by heat treatment.