Experimental and theoretical studies of the equilibrium and dynamics of the penetration of the soluble cationic surfactant cetyltrimethylammonium bromide (CTAB) into arachidic acid monolayers are reported. The experimental studies of the system are performed at different pH values in acidic, alkaline, and neutral media. The penetration of CTAB from the alkaline subphase into the arachidic acid monolayer is shown to result in a significant increase of the surface pressure of the mixed monolayer, whereas the surface pressure jump for the penetration from the acidic subphase does not differ from that corresponding to the adsorption from the single CTAB solution with the same concentration onto the pure aqueous surface. This phenomenon is explained by the fact that the dissociation degrees of arachidic acid are different (the dissociation is the largest in the alkaline subphase, and nearly no dissociation exists in the acidic subphase), and in the alkaline subphase, strong Coulombic interaction exists between the CTA(+) cation and the arachidate anion in the mixed monolayer. A new theoretical model is proposed involving the enthalpic nonideality of the mixed monolayer (the interaction between the insoluble and soluble monolayer components) which provides a quantitative interpretation of the experimental results with physically reasonable values of the model parameters.