Fumed mixed silica-titania (FM-Si/Ti) core materials and their corresponding hydrophobized counterparts are compared with respect to their applicability as external additives for electrophotographic toners using toner flowability and Tribo-Electrostatic Charge (T-ESC) stability under extended activation periods as well as L/L and H/H conditions. Flowability studies, which included angle of repose and tapped density measurements, showed differences in the toner flow enhancement with titania and FM-Si/Ti compared to silica alone, both under static and dynamic conditions. The influence of silica in FM-Si/Ti on the initial charge-up behavior was also observed in T-ESC measurements. The differences were explained by TEM investigations, indicating the importance of the silica/titania ratio of the nano-particle structure. In addition, volume resistivity (Ω·cm) studies showed that the structure strongly relates to the electro-physical properties of the FM-Si/Ti. The influences of a thin, encapsulating silica layer on the titania core of mixed oxides are described in detail.