The aliphatic n-butyr- and n-valeraldehyde as well as the aromatic benz- and anisaldehyde induced DNA strand breaks in PM2 DNA in the presence of CuCl2. Neither aldehydes nor CuCl2alone showed DNA breakage properties. The maximum of single strand breaks (SSBs) induced by the combination of CuCl2and aldehydes was dependent on the CuCl2-concentration. The aliphatic aldehydes induced SSBs and double strand breaks (DSBs) at lower concentrations than aromatic aldehydes when optimal CuCl2concentration were used. Catalaseandneocuproine nearly completely inhibited strand break formation induced by aromatic aldehydes/CuCl2.The prevention of strand breaks induced by aliphatic aldehydes/CuCl2was less effective. While the inhibition by neocuproine was only 25 %, catalase was totally ineffective. In all aldehydes/CuCl2mixtures the formation of Cu(I) was observed. The results point to different DNA damaging species produced during redox reactions of aromatic and aliphatic aldehydes in combination with CuCl2.