Monovalent silver ions, resulting from a thin silver layer initially deposited at the anode surface, have been introduced using the field-assisted ion-exchange technique in sodium niobium borophosphate glasses. A reproducible susceptibility χ⁽²⁾ could be gained after this poling treatment, although a drop in the nonlinearity is observed due to the introduction of silver ions. From energy dispersive x-ray spectroscopy, it has been found that the nonlinear layer is characterized by a strong migration of sodium ions 4 μ m deep inside the anode side, which have been partially replaced by silver ions. These results indicate a complex space-charge-migration process during the poling treatment, which is involved in the decrease in the mean second-harmonic generation signal.