The electric field recorded in silica samples thermally poled during various intervals was measured by etching. The field is nearly uniform and increases to a maximum (∼3.6×108 V/m) in few minutes of poling and subsequently decreases slowly toward a steady level (∼1.4×108 V/m). The depletion layer becomes neutral even before the field reaches steady state. An upper limit of (0.7±0.2) μm was found for the thickness of the negatively charged edge of the depletion region. The value of χ(3) is the same for all poling times.