Low molecular surface-active substances such as fatty acids and alcohols have a detrimental effect on head retention. However, a quantitative analysis of their detrimental effect on head retention has not been reported. The low molecular surface-active substances weaken the foam films by adsorbing on the bubble surface or on major foam generating proteins. The detrimental effect on head retention caused by the various fatty acids was quantitatively demonstrated. That is, the detrimental effect and the defoaming mechanism due to the various fatty acids are related to their adsorbing properties on the surface. Surface excess is known as a physical parameter for the molecular adsorptive property. It is defined as how much solute adsorbs at the gas/liquid interface at certain bulk concentrations. Its units are expressed as mol/m(^2). If the surface excess of a substance per its unit bulk concentration is known, the surface excess at a certain bulk concentration can be calculated (adsorbing amount of the substance per unit foam cell area). The surface excess of various fatty acid solutions was determined, and then the relation between the detrimental effect on head retention by the fatty acids and surface excess was examined. The head retention value did not depend on the kind of fatty acids, and it had a negative correlation to the surface excess. Moreover, when more than one fatty acid was dissolved in beer, the total surface excess of the fatty acids showed a negative correlation toward the head retention value. The surface excess of several fatty acids (n-caproic acid (C6:0), n-caprylic acid (C8:0), n-capric acid (C10:0), lauric acid (C12:0) and palmitic acid (C16:0) ) were measured. The surface excess per its unit bulk concentration exponentially increased along with the crabon number. Moreover, the relationship between the hydrophobicity and the surface excess of the tested fatty acids could be expressed by an exponential function. This indicated that the adsorbing amount of palmitic acid on the foam cell surface is 30 times greater than the amount of caprylic acid when both acid concentrations are the same. The NIBEM values were determined for the fatty acid-added beers. The NIBEM values of the tested beers linearly decreased along with the surface excess. We described the quantitative analytical technique for the relations between the concentration of various surface-active low molecular substances, its surface excess and the detrimental effect on head retention by such surface-active substances. We believe that the relations that we have described provide a strong clue when we analyze various beer foam qualities.