The structure of DNA molecules condensed on cationic lipid bilayers changes in the presence of high-valence cations. For a plasmid DNA, it is found that the interhelical distance of condensed DNA has a minimum as the concentration of Mg2+ ions increases. However, divalent Mn2+ ions do not induce similar changes. More dramatically, the triamine spermidine causes most condensed DNA molecules to leave the bilayers at a concentration of only 0.05 mM. These findings may be useful for understanding how 2-D condensation of DNA occurs.