Pyrogenic silicas modified with groups of the general formula -O 1+n -Si(OR 1) 2-n -R 2 -NR 3 2 in which n may be 0, 1, 2, in which R 1 may be a hydrogen atom or a C-O-bonded C 1 -C 15 -hydrocarbon radical, or an acetyl radical, R 2 may be methylene group (-CH2-), ethylene group (-C2H4-), 1,3-propyl group (-C3H6-) or phenylene group (-C6H4-), R 3 a hydrogen atom or an optionally mono- or polyunsaturated N-C-bonded, monovalent, optionally divalent, C 1 -C 20 -hydrocarbon radical optionally substituted by -CN, -NCO, -NR 4 2, -COOH, -COOR 4, -halogen, -acryloyl, -epoxy, -SH, -OH or -CONR 4 2, or an aryl radical, or a C 1 -C 15 -hydrocarbon-oxy radical in which in each case one or more, non-neighboring methylene units may be replaced by groups -O-, -CO-, -COO-, -OCO- or -OCOO-, -S- or -NR 4 - and in which one or more, non-neighboring methine units may be replaced by groups -N=, -N=N- or -P=, it being possible for R 3 to be identical or different or for the amine group -N(R 3) 2 also to be part of an aliphatic or aromatic heterocycle, R 4 has the same meaning as R 1 and R 4 may be identical or different, characterized in that they have a positive triboelectric surface charge q trib of from +50 μC/g to +1000 μC/g and a relative decrease in the triboelectric surface charge (q trib - q trib) × 100%/q trib of not more than 60%, q trib being the triboelectric surface charge after an activation time of 3600 s and q trib being the triboelectric surface charge after an activation time of 150 s of a mixture of a toner powder and the metal oxide according to the invention and it being necessary for q trib to be greater than 0, the mixture having been stored at 32°C and 80% relative humidity for 5 d prior to the measurement, the triboelectric surface charge and the relative decrease thereof having been determined as defined in the description, the proportion of the sum of the NMR signal intensities of the T 1 and T 2 groups, based on the total intensity of the T groups T 1 + T 2 + T 3, being at least 20%, the T groups being defined as T 1: R-Si(OR 1) 2 -O-Si T 2: R-Si(OR 1)(-O-Si) 2 T 3: R-Si(-O-Si) 3, it being possible for the relative proportion of the T 1, T 2 and T 3 groups to be determined by solid-state NMR spectroscopy in the 29 Si-CPMAS mode, R being any desired organic group and R 1 being a hydrogen atom or a C-O-bonded C 1 -C 15 -hydrocarbon radical, or an acetyl radical, preparable in a process in which the modification of the metal oxides takes place under anhydrous conditions with organosilanes of the general formula II X 1+n Si (OR 1) 2-n -R 2 -NR 3 2 in an amount of 1% by weight to 20% by weight based on the silica, per hydrophilic silica surface area of 100 m 2 /g BET surface area used, R 1, R 2, R 3, R 4 and n being as defined above, it being possible for R 1 and R 4 to be identical and X = halogen, nitrogen radical, OR 4, OCOR 4, O(CH 2) X OR 4 and x = 1, 2, 3, the coating of the silica taking place at temperatures of from 20 to 80°C, reaction being effected with a temperature gradient in the range from 40°C to 1800°C, this meaning that the product temperature at the beginning of the reaction is in a range of 40-100°C and furthermore that the product temperature toward the end of the reaction is in a range of 80-180°C, with the proviso that the product temperature at the beginning of the reaction is lower than toward the end of the reaction, the reaction temperature being increased stepwise and the residence time per reaction temperature being between 5 min and 240 min, and the temperature gradient being dependent on the location (continuous) or dependent on the time (batchwise) and the purification taking place at a purification temperature of from 20 to 200°C.