The interfacial tensions of lauric and myristic acid sodium salt water solutions were measured at the air/water interface at 20 degrees C and at the air/0.01 M NaOH water solution interface at 20 degrees and 40 degrees C. On the basis of these results the dependence of standard free energy of adsorption (Delta G(0)) on the interface coverage (Theta) was determined from the Blomgren-Bockris isotherm. According to these dependencies the interactions of adsorbed molecules were discussed and dielectric constants of the adsorbed monolayer were estimated.