Starting from an equation similar to that proposed by Volmer and Frumkin for the surface pressure of neutral long chain molecules, an equation has been derived for the surface pressure of long chain electrolytes at oil/water interface taking electrostatic repulsion and interionic attraction into consideration. The equation may be written as follows π=2n0kT In \(({A{} \over A-A_0})\)+(exp (A-A0)a/A-1) n0kT In +\({n_0kT{} \over 2}\) ln f where n= \({1{} \over A}\) represents the number of long chain ions adsorbed per unit area of the interface, f their mean activity coefficient and A0= \({1{} \over n_0}\) the other terms have their usual significance. Experimental evidence available in the literature has been cited in support of this equation.