Gegenion complex, Langmuir monolayers at the air-water interface, composed of poly(ethyleneimine) (PEI(70000)) and arachidic acid, ranging between pH 2 and pH 12, were prepared and the effect of the pH on the forming of the complex monolayer was investigated. Gegenion complex, Langmuir-Blodgett film depositions of varying pH on ZnSe wafers and transmission IR of the samples allowed analysis of the composition of the Langmuir-Blodgett film. The surfactant and polymer adhere to one another by means of protonation and deprotonation of the charged head group of arachidic acid and amine groups of PEI(70000). By analyzing how the pH affects the interaction between the surfactant and the polymer, an optimum pH near 8 was determined, in which both the fatty salt and the polymer form the Gegenion complex, Langmuir layer at the air-water interface. For pH greater or less than 8, IR results of Gegenion complex, LB films show decreased adhesion of the polymer to the surfactant.