Monolayers of simple fatty acids exhibit a rich polymorphism with at least seven solid phases. The reason for such a variety of phases is not known yet and therefore is the object of the current investigation. In the present work the pH of the subphase is varied between 2 and 10.5 and the influence of the degree of dissociation on phase transition pressures and temperatures of arachidic acid monolayers is investigated in detail in the range from 2 to 45 degreesC by means of Brewster-angle microscopy and surface pressure-area isotherms. The shift of phase boundaries is ascribed to the change in the steric, electrostatic, and bonding interactions between the lipid headgroups. The impressive correlation between the L-2/v nearest neighbor to next nearest neighbor (NN/NNN) transition pressure and the position correlation length as functions of the pH value suggests that the L-2/Ov NN/NNN transition in fatty acid monolayers is strongly influenced or even originates in headgroup bonding interactions. The influence of headgroup dissociation on the phase behavior of fatty acid monolayers is investigated in the absence of solutes that interact with the lipids. The comparison of the phase 'behavior of this system with that of a system, in which solutes are present that interact with the charged fatty acids, e.g., polymers or polyvalent metallic ions, should provide information on specific solute-lipid interactions separately from the effect of headgroup dissociation.