In the presence of counterions, the quality of fatty acid monolayers, prepared by the Langmuir-Blodgett (LB) technique is highly influenced by the subphase pH value. In this paper, we present ellipsometric studies on arachidic acid LB films, prepared at different subphase pH values, with uranyl (O=U=O)(2+) as the counterion. In order to obtain the refractive indices and thickness, independently, the films were prepared as steps with bilayer step size. Refractive index, real and imaginary part, monolayer thickness and surface roughness on a microscopic and macroscopic scale have been determined for three different pH values. The results show that films prepared at a pH value of 4.8 and 4.0 are of good optical quality, whereas those prepared at pH 2.5 are inhomogeneous.