The interaction of DNA with monolayers of the cationic lipid dimethyldioctadecylammonium bromide, with/without 50 mol % of a neutral "helper" lipid, either dioleoylphosphatidylethanolamine or cholesterol, has been studied using specular neutron reflection, surface pressure-area isotherms, and Brewster angle microscopy. The amount of DNA bound to the lipid head groups has been comprehensively quantified in the range of 8-39 vol% of DNA with respect to the monolayer composition (monolayers composed of dimethyldioctadecylammonium bromide binding the most DNA and monolayers containing dioleoylphosphatidylethanolamine binding the least) and surface pressure (DNA binding being greatest at highest surface pressures). Surprisingly, regardless of these variables, the thickness of the DNA-containing layer remained approximately constant between 18 and 25 angstrom. This systematic study is the first direct quantification of the binding of DNA with two different helper-lipid-containing multicomponent monolayers, an important step toward understanding interaction parameters in more realistic models of gene delivery systems.