Aqueous dispersions of polymers which obtained by free radical suspension polymerization or free radical miniemulsion polymerization of ethylenically unsaturated monomers in an oil-in-water emulsion whose disperse phase comprises at least one fluorescent dye dissolved in at least one ethylenically unsaturated monomer and has an average particle diameter of at least 1μm, in the presence of at least one surface-active compound and at least 0.5% by weight, based on the monomers, of at least one hydrophobic, nonpolymerizable, organic compound, of a hydrophobic polymer of at least one C2- to C6-olefin having a molar mass Mw of up to 10000, of a siloxane having a molar mass Mw of up to 5000 and/or polystyrene having a molar mass Mw of up to 10000, and of the powders obtainable from these polymer dispersions in each case by drying and comprising at least one fluorescent dye for marking of materials.