A process for preparing a surface treated filler material product with mono-substituted succinic anhydride(s) and a mixture of aliphatic linear or branched carboxylic acids comprising stearic acid, the process comprising at least the steps of: a) providing at least one calcium carbonate-containing filler material, b) providing at least one mono-substituted succinic anhydride, c) providing a mixture of aliphatic linear or branched carboxylic acids comprising stearic acid in an amount of at least 10.0 wt.-%, based on the total weight of the mixture, and one or more further saturated aliphatic linear or branched carboxylic acid(s) having a total amount of carbon atoms from C8 to C24, d) contacting the surface of the at least one calcium carbonate-containing filler material of step a), under mixing, in one or more steps, in any order, with the at least one mono-substituted succinic anhydride of step b) and the mixture of aliphatic linear or branched carboxylic acids of step c) such that a treatment layer comprising the at least one mono-substituted succinic anhydride and/or salty reaction product(s) thereof and the mixture of aliphatic linear or branched carboxylic acids and/or salty reaction product(s) thereof is formed on the surface of said at least one calcium carbonate-containing filler material of step a), wherein the temperature before and/or during contacting step d) is adjusted such that the at least one mono-substituted succinic anhydride and the mixture of aliphatic linear or branched carboxylic acids is in a molten or liquid state and wherein contacting step d) is carried out in that the at least one mono-substituted succinic anhydride of step b) and the mixture of saturated aliphatic linear or branched carboxylic acids of step c) are added in a weight ratio [succinic anhydride/ mixture of carboxylic acids] of from 10:1 to 1:10 and wherein the at least one mono-substituted succinic anhydride of step b) is added in contacting step d) in a total amount of from 0.1 to 3 wt.-%, based on the total dry weight of the at least one calcium carbonate-containing filler material of step a); and the mixture of saturated aliphatic linear or branched carboxylic acids of step c) is added in contacting step d) in a total amount of from 0.1 to 3 wt.-%, based on the total dry weight of the at least one calcium carbonate-containing filler material of step a). a) providing at least one calcium carbonate-containing filler material, b) providing at least one mono-substituted succinic anhydride, c) providing a mixture of aliphatic linear or branched carboxylic acids comprising stearic acid in an amount of at least 10.0 wt.-%, based on the total weight of the mixture, and one or more further saturated aliphatic linear or branched carboxylic acid(s) having a total amount of carbon atoms from C8 to C24, d) contacting the surface of the at least one calcium carbonate-containing filler material of step a), under mixing, in one or more steps, in any order, with the at least one mono-substituted succinic anhydride of step b) and the mixture of aliphatic linear or branched carboxylic acids of step c) such that a treatment layer comprising the at least one mono-substituted succinic anhydride and/or salty reaction product(s) thereof and the mixture of aliphatic linear or branched carboxylic acids and/or salty reaction product(s) thereof is formed on the surface of said at least one calcium carbonate-containing filler material of step a), wherein contacting step d) is carried out in that the at least one mono-substituted succinic anhydride of step b) and the mixture of saturated aliphatic linear or branched carboxylic acids of step c) are added in a weight ratio [succinic anhydride/ mixture of carboxylic acids] of from 10:1 to 1:10 and wherein the at least one mono-substituted succinic anhydride of step b) is added in contacting step d) in a total amount of from 0.1 to 3 wt.-%, based on the total dry weight of the at least one calcium carbonate-containing filler material of step a); and the mixture of saturated aliphatic linear or branched carboxylic acids of step c) is added in contacting step d) in a total amount of from 0.1 to 3 wt.-%, based on the total dry weight of the at least one calcium carbonate-containing filler material of step a).