The invention relates to nano- or microparticles containing an active substance embedded in a polymer matrix and to a method for their production, comprising the steps of: a) effecting precipitation of an active substance in a solution which comprises a polymer dissolved in an organic solvent to obtain a suspension of the active substance, b) mixing the obtained suspension with an aqueous surfactant solution and solidifying the polymer to obtain a suspension of nano- or microparticles which contain an active substance.
