This work is focused on the enhancement of the monolayer stability by studying the pH-dependent adsorption of metal ions of varying valence to a stearic acid monolayer. The stability of the monolayer was detected by measuring the time-dependent change in the mean molecular area at a constant surface pressure. A rather narrow ion-specific pH range of maximum stability was found. Within this pH range, the monolayer acted almost as an ideal film and the molecular area at a predetermined surface pressure remained constant for several hours. The stability was correlated with the degree of dissociation of the monolayers which was determined by X-ray photoelectron spectroscopy. The observed pK(a) values were 4.3 for Tb stearate, 5.6 for Cd stearate and 6.2 for Mn stearate. The deposited multilayer structures were analyzed by atomic force microscopy. The films deposited at a pH of poor stability clearly contained more defects than the films fabricated at a pH of maximum stability. The resolution of the images was sufficient for a detailed crystallographic analysis of the molecular packing.