Abstract : The local chromatin organization and the processes leading to its dense packing are still puzzling. We aim at understanding the role of local charges and ionic concentrations variations on the chromatin supramolecular organization. We have focused our studies on the repeated chromatin units, which are the nucleosome core particles (NCPs), made of 146bp DNA wrapped around a histone octamer. Histone terminal extensions ("tails") are mobile and highly positively charged. NCPs were reconstituted from recombinant DNA and full-length or truncated histones to demonstrate that removal of both H3 and H4 histone tails prevents attractive interactions to occur, in dilute solution. Phase diagrams of NCPs precipitated by multivalent cations (Mg2+ and Spd3+) were designed. The precipitated phases structure was analyzed. The role of H3 and H4 histone tails in the dense phase formation, obtained under osmotic stress, was analyzed, using micro-volumes of reconstituted recombinant NCPs.