Abstract The monolayer characteristics of behenic acid upon subsolutions from pH 2 to 14 are presented. The influence of atmospheric CO 2 on these characteristics has been investigated ( 1) by performing experiments in a CO 2-free atmosphere and ( 2) by using various buffers. CO 2 absorbed into the surface region is shown to exert appreciable buffering activity; this results in an increase in the pH of expansion of the monolayer and an increase in the surface potential ( ΔV) in the pH range 6–10.5 for unbuffered systems. The corrected ΔV-pH curve differs appreciably from previous curves obtained with buffered solutions. A theoretical ΔV-pH curve, based on the Gouy model and on dissociation via the acid-soap species, is in good agreement with the experimental curve. The surface moment of the car☐ylate ion was estimated for the calculations.