The invention relates to aqueous fungicidal active substance compositions and to their use in the control of harmful microorganisms and in particular in the protection of cellulose-comprising materials, particularly wood, from infection by microorganisms, in particular those harmful fungi which can damage wood or cellulose.The active substance composition according to the invention comprises: a) at least one fungicidal organic active substance with a solubility in water of not more than 5 g/l at 25° C./1013 mbar, and b) a finely-divided polymer with an average particle size, determined by dynamic light scattering, of not more than 300 nm, in which the polymer particles comprise the active substance, the polymer being formed from ethylenically unsaturated monomers M comprising: at least 60% by weight, based on the total amount of the monomers M, of at least one neutral monoethylenically unsaturated monomer M1 with a solubility in water of not more than 30 g/l at 25° C., and up to 40% by weight, based on the total amount of the monomers M, of one or more ethylenically unsaturated monomers M2 other than the monomers M1.