The toxic effects of new quaternary ammonium chlorides on dicotyledonous plant Sinapis alba was evaluated. Investigated compounds differed either in hydrophobic chain length or hydrophilic part properties. The compounds used in the study are meant to be new wood preservatives because of their excellent fungicidal properties. It has been stated that the toxic effects of examined chlorides on Sinapsis alba depend on the compound structure. Alkyl(alkoxymethyl)dimethylammonium chlorides had the lowest phytotoxic properties. The presence of benzene subsituent in the compound molecule caused an increase in toxic effects of ammonium chlorides on the Sinapis alba roots.