The Gibbs adsorption formula for a binary mixture of liquids, γ2= γ1(x/(1 -x)) - (x/RT)δγ/δx, and Guggenheim's formula,A1γ1+A2γ2= 1, are examined in a practical case, the surface of a liquid mixture of adiponitrile and tris(dioxa-3,6-heptyl)amine (γ1and γ2are the surface concentrations of the two components,xis the bulk mole fraction of component 2,Ris the gas constant,Tis the temperature,A1andA2are the area of the components per monolayer of unit surface area and γ is the surface tension). Loci of constant values ofE= (γ2/γ1)/(x/(1 -x)) are plotted in the planeA1,A2. Such loci (1) are linear (2) pass through the pointA1*,A2* =RT/(δγ/δx(1 -x)), -RT/(x(δγ/δx)) and (3) possess a linear scale parallel to theA1axis. Measurements of γ1and γ2carried out by He(I) photoelectron spectroscopy support the above better if it is assumed that not one butNmonolayers are sampled by the technique, withA1γ1+A2γ2=N.The value ofNis found to be between 1 and 2.