The thermodynamic and morphological properties of arachidic acid monolayers at high pH values (pH 11-13) and temperatures (10 degrees C less than or equal to T less than or equal to 35 degrees C) are studied. The two-phase coexistence region of the surface pressure (Pi)-area (A) isotherms is shifted to higher surface pressures with increasing pH value, just as with increasing temperature. Accordingly, regularly shaped condensed phase domains are formed which have various textures depending on the pH of the aqueous subphase. The equation of state, derived recently using the generalized Volmer's equation and the quasichemical equilibrium model of 2D aggregation, was developed for the case of dissociating monolayer material taking into account the effects caused by the increase in the number of kinetic entities from the dissociation of ions and a contribution to the surface pressure, related to the ion-ion interaction. The experimental Pi-A isotherms can be well described over the entire region of the gaseous and condensed states by the equation of state; The results allow the conclusion that the dissociation of arachidic acid in the monolayer increases with increasing pH, does not exceed 15%, and can be neglected for pH less than or equal to 12. The thermodynamic characteristics of the aggregation process of arachidic acid in the monolayer, particularly the negative value of standard enthalpy of aggregate formation and that of standard entropy of aggregation, indicate the formation of a highly ordered condensed phase at the expense of the nonordered fluid phase.