n-Octadecyl ammonium chloride is spread on the subphase containing different kinds of metal ions and small molecule salts[CH3NH+3Cl-,(CH3)4N+Cl-,(C2H5)4N+Cl-,(C4H9)4N+Cl-].Surface pressure-area isotherm curves are measured under the appropriate conditions to characterize properties of the molecular monolayers.The influences of small molecule salts on ODH Langmuir films are investigated.The results show that radius of the metal ions,steric configuration of small molecule salt,the electro-negativities of the elements and hydrophobic/hydrophilic interactions of the complexes have important effects on the properties of monolayer films.