The stability of arachidic acid monolayers on water at different pH values and on aqueous solutions of 10 mM monovalent (NaCl and LiCl) or divalent (CaCl2 and MgCl2) salts has been studied. Relaxation measurements of the monolayer kept at a constant surface pressure (from Π=10 to Π=50 mN/m) reveal that the presence and the nature of the cation dramatically influence the rate of collapse. The order of increasing stability is H+ < Li+ < Na+ < Ca2+ < Mg2+ . The determination of equilibrium spreading pressures shows that in all cases the monolayer is thermodynamically unstable at surface pressures above 0.25mN/m. The correlation between the order of stability and the calculated degree of dissociation of the monolayer suggests that electrostatic forces within or between nuclei formed in the film may create an important stabilizing effect for monolayers on aqueos subphases containing monovalent electrolyte. The much greater stability induced by divalent counterions can, however, hardly be rationalized in terms of the ionization of the monolayer.