The polarization response of the Buried OXide (BOX) in UNIBOND Fully Depleted MOSFETs is investigated by means of a modified drain current DLTS setup. The investigation is based on the transient analysis that indicates the presence of dipolar relaxation as well as polarization processes related to interface charge trapping. The BOX dielectric relaxation shows the presence of both Debye and complex relaxation processes. The coupling between gate modulation and BOX polarization is also considered.