Aqueous sizing compositions for optical brightening of substrates, preferably paper, comprising a) at least one optical brightener of formula in which the anionic charge on the brightener is balanced by a cationic charge composed of one or more identical or different cations selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 -C 4 linear or branched alkyl radical, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 -C 4 linear or branched hydroxyalkyl radical, or mixtures of said compounds, R 1 and R 1 ' may be the same or different, and each is hydrogen, C 1 -C 4 linear or branched alkyl, C 2 -C 4 linear or branched hydroxyalkyl, CH 2 CO 2 -, CH 2 CH 2 CONH 2 or CH 2 CH 2 CN, R 2 and R 2 ' may be the same or different, and each is C 1 -C 4 linear or branched alkyl, C 2 -C 4 linear or branched hydroxyalkyl, CH 2 CO 2 -, CH(CO 2 -)CH 2 CO 2 -, CH(CO 2 -)CH 2 CH 2 CO 2 -, CH 2 CH 2 SO 3 -, CH 2 CH 2 CO 2 -, CH 2 CH(CH 3)CO 2 -, benzyl, or R 1 and R 2 and/or R 1 ' and R 2 ', together with the neighboring nitrogen atom signify a morpholine ring and p is 0, 1 or 2, b) at least one dye of formula in which R 3 signifies H, methyl or ethyl, R 4 signifies paramethoxyphenyl, methyl or ethyl, M signifies a cation selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium, ammonium which is mono-, di, tri- or tetrasubstituted by a C 1 -C 4 linear or branched alkyl radical, ammonium which is mono, di-, tri- or tetrasubstituted by a C 1 -C 4 linear or branched hydroxyalkyl radical, or mixtures of said compounds, (c) at least one binder, (d) optionally one or more divalent metal salts and (e) water. a) at least one optical brightener of formula in which the anionic charge on the brightener is balanced by a cationic charge composed of one or more identical or different cations selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 -C 4 linear or branched alkyl radical, ammonium which is mono-, di-, tri- or tetrasubstituted by a C 1 -C 4 linear or branched hydroxyalkyl radical, or mixtures of said compounds, R 1 and R 1 ' may be the same or different, and each is hydrogen, C 1 -C 4 linear or branched alkyl, C 2 -C 4 linear or branched hydroxyalkyl, CH 2 CO 2 -, CH 2 CH 2 CONH 2 or CH 2 CH 2 CN, R 2 and R 2 ' may be the same or different, and each is C 1 -C 4 linear or branched alkyl, C 2 -C 4 linear or branched hydroxyalkyl, CH 2 CO 2 -, CH(CO 2 -)CH 2 CO 2 -, CH(CO 2 -)CH 2 CH 2 CO 2 -, CH 2 CH 2 SO 3 -, CH 2 CH 2 CO 2 -, CH 2 CH(CH 3)CO 2 -, benzyl, or R 1 and R 2 and/or R 1 ' and R 2 ', together with the neighboring nitrogen atom signify a morpholine ring and p is 0, 1 or 2, b) at least one dye of formula in which R 3 signifies H, methyl or ethyl, R 4 signifies paramethoxyphenyl, methyl or ethyl, M signifies a cation selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium, ammonium which is mono-, di, tri- or tetrasubstituted by a C 1 -C 4 linear or branched alkyl radical, ammonium which is mono, di-, tri- or tetrasubstituted by a C 1 -C 4 linear or branched hydroxyalkyl radical, or mixtures of said compounds, R 3 signifies H, methyl or ethyl, R 4 signifies paramethoxyphenyl, methyl or ethyl, M signifies a cation selected from the group consisting of hydrogen, an alkali metal cation, alkaline earth metal, ammonium, ammonium which is mono-, di, tri- or tetrasubstituted by a C 1 -C 4 linear or branched alkyl radical, ammonium which is mono, di-, tri- or tetrasubstituted by a C 1 -C 4 linear or branched hydroxyalkyl radical, or mixtures of said compounds, (c) at least one binder, (d) optionally one or more divalent metal salts and (e) water.