The invention relates to a process for reducing the minimum inhibitory concentration (MIC) of a biocide against at least one strain of bacteria and/or at least one strain of yeast and/or at least one strain of mould in an aqueous preparation. The invention further relates to the use of a water soluble source of lithium ions for reducing the minimum inhibitory concentration (MIC) of a biocide against at least one strain of bacteria and/or at least one strain of yeast and/or at least one strain of mould in an aqueous preparation.