We studied the compaction of DNA in a crowding environment on poly(ethylene glycol) solution induced by the addition of salt, NaCl or Na2SO4, through single-DNA observation with particular focus on the effect of co-ions. We found that, at the same concentration of monovalent cation, divalent anions retard compaction more than monovalent anions, which means that greater ionic strength is not favorable for compaction. We discuss this effect in relation to the change in translational entropy of small ions accompanied by the breakdown of the physico-chemical framework of the screening ion atmosphere upon compaction. (c) 2008 Elsevier B.V. All rights reserved.