The sensitivity of octadecanoic acid (stearic acid, SA) monolayers and of three unsaturated derivatives thereof toward MnCl2, CdCl2, and TbCl3 introduced to the subphase has been investigated as a function of electrolyte concentration. The unsaturated fatty acids studied were cis-9-octadecenoic acid (oleic acid, GA), trans-6-octadecenoic acid (petroselaidic acid, PA), and trans-9-octadecenoic acid (elaidic acid, EA). The order of condensation of the monolayers followed the bulk melting points of the acids. The dissociation degree determined for multilayers by attenuated total reflectance Fourier transform IR in excellent agreement with the results obtained for the floating monolayers. The metal ions could be ordered in terms of increasing interaction with the monolayers as Mn2+ < Cd2+ << Tb3+. Already a TbCl3 content of 10(-6) M was enough to totally convert a stearic acid monolayer to the stearate form. The monolayer behavior of PA and EA was similar to that of SA. OA could not be compressed to its equilibrium spreading pressure (ESP) with Tb3+ present in the subphase at normal pH (5.7). However, if the pH was raised to 6.8, a solidlike phase was apparent in the isotherm, and the monolayer could be compressed to surface pressures exceeding the ESP of the pure acid. This feature was not observed for the other metal ions. Good quality Y-type deposition of fully dissociated SA, PA, and EA monolayers was obtained.