The use of additives selected from the group consisting of a1) organic acids having a pKa of below 4.2, selected from the group consisting of a1a) aromatic sulfonic acids and a1b) singularly or doubly alkoxy-, mercapto- or alkylmercapto-substituted alkanecarboxylic acids having two carbon atoms, singularly or doubly halogen-, alkoxy-, mercapto- or alkylmercapto-substituted alkanecarboxylic acids, alkenedicarboxylic acids or alkanedicarboxylic acids having at least three carbon atoms, a2) phosphites of the formula where R is preferably an aryl group which is substituted in positions 2, 4 and 6 as follows: Position 2: tert-butyl, tert-amyl, Position 4: hydrogen, alkyl, tert-butyl or tert-amyl, and Position 6: hydrogen, alkyl, tert-butyl or tert-amyl, with the proviso that at least one of the substituents in positions 4 and 6 is not hydrogen, a3) phosphonites of the formula 2 P-X-P 2 where R is preferably an aryl group which is substituted in positions 2, 4 and 6 as follows: Position 2: tert-butyl, tert-amyl, Position 4: hydrogen, alkyl, tert-butyl or tert-amyl, and Position 6: hydrogen, alkyl, tert-butyl or tert-amyl, with the proviso that at least one of the substituents in positions 4 and 6 is not hydrogen, and X in this case is an arylene group, a4) acidic phosphorus derivatives selected from the group consisting of a4a) mono- and di-C 1 to C 12 alkyl phosphates, a4b) mono- and di-C 1 to C 12 alkyl phosphonates, a4c) mono-C 1 to C 12 alkyl phosphinates, and a4d) alkyl derivatives of phosphorus-containing diacids, for reducing flocculation and/or precipitation on exposure to (atmospheric) moisture during storage of polyisocyanate mixtures which comprise at least one solvent. a1) organic acids having a pKa of below 4.2, selected from the group consisting of a1a) aromatic sulfonic acids and a1b) singularly or doubly alkoxy-, mercapto- or alkylmercapto-substituted alkanecarboxylic acids having two carbon atoms, singularly or doubly halogen-, alkoxy-, mercapto- or alkylmercapto-substituted alkanecarboxylic acids, alkenedicarboxylic acids or alkanedicarboxylic acids having at least three carbon atoms, a2) phosphites of the formula where R is preferably an aryl group which is substituted in positions 2, 4 and 6 as follows: Position 2: tert-butyl, tert-amyl, Position 4: hydrogen, alkyl, tert-butyl or tert-amyl, and Position 6: hydrogen, alkyl, tert-butyl or tert-amyl, with the proviso that at least one of the substituents in positions 4 and 6 is not hydrogen, Position 2: tert-butyl, tert-amyl, Position 4: hydrogen, alkyl, tert-butyl or tert-amyl, and Position 6: hydrogen, alkyl, tert-butyl or tert-amyl, a3) phosphonites of the formula 2 P-X-P 2 where R is preferably an aryl group which is substituted in positions 2, 4 and 6 as follows: Position 2: tert-butyl, tert-amyl, Position 4: hydrogen, alkyl, tert-butyl or tert-amyl, and Position 6: hydrogen, alkyl, tert-butyl or tert-amyl, with the proviso that at least one of the substituents in positions 4 and 6 is not hydrogen, and X in this case is an arylene group, Position 2: tert-butyl, tert-amyl, Position 4: hydrogen, alkyl, tert-butyl or tert-amyl, and Position 6: hydrogen, alkyl, tert-butyl or tert-amyl, a4) acidic phosphorus derivatives selected from the group consisting of a4a) mono- and di-C 1 to C 12 alkyl phosphates, a4b) mono- and di-C 1 to C 12 alkyl phosphonates, a4c) mono-C 1 to C 12 alkyl phosphinates, and a4d) alkyl derivatives of phosphorus-containing diacids, for reducing flocculation and/or precipitation on exposure to (atmospheric) moisture during storage of polyisocyanate mixtures which comprise at least one solvent.