Brewster angle microscopy (BAM) studies on dipalmitoyl phosphatidyl glycerol (DPPG, sodium salt) monolayers on water subphases containing different amounts of NaCl are presented. Inhomogeneities in the monolayer appear after spreading at large mean areas per lipid molecule and at electrolyte concentrations in the subphase lower than 0.15 M NaCl. IL is assumed that (i) after spreading DPPG is completely hydrolyzed, (ii) dissociation of the resulting acid depends on the electrolyte concentration in the subphase, and (iii) nondissociated acid molecules form solidlike aggregates in the monolayer, while the dissociated part surrounds them. Image processing of the BAM images allows us to calculate the surface densities of the lipid in the coexisting states, and the dissociation constant of the lipid in the monolayer (K-d(s)). The electrostatic contribution to the surface pressure was estimated using the obtained K-d(s) value. Comparison with the experimental pi/A isotherms shows satisfactory agreement, confirming the value of the determined dissociation constant. We concluded that the described procedure is useful for the determination of K-d(s) for monolayers of charged surfactant molecules.