The ionic charge dependence of a lipid-polymer has been studied at an air/aqueous solution interface. The surface pressure and surface area/molecule isotherms of DMPE-PEG5000 system were measured on the aqueous solution containing KCl, NaCl, or CaCl2 with a several different salt concentration by using the Langmuir Surface Balance technique. The two dimensional equation of states of a lipid-polymer was also calculated theoretically to compare the experimental results on the electrolyte subphase. We found that the isotherms on the electrolyte solutions vary relatively small in comparison with an isotherm on the water, which is the inside of the experimental error of the instrument. Therefore, we conclude that the salt effects are very small, which is reasonable based on the osmotic pressure of high molecular weight PEG moiety. We recommend the construction of a high precision surface balance if the investigation of the small salt effects at a DMPE-PEG monolayer on the electrolyte solution is needed.