Titration of a series of C-18 fatty acids yields pK(a) values that decrease with an increasing degree of unsaturation in the fatty acid chain. The pKa values of stearic, elaidic, oleic, linoleic, and linolenic acids were studied and compared to values of area per molecule in a spread monolayer of these acids. The decrease in pK(a) was found to relate to melting point temperature and area per molecule in the spread fatty acid monolayer. The pKa value was determined by first dissolving the fatty acid in a high pH solution (pH > 10) and subsequently titrating the solution with HCl to obtain the characteristic S-shaped curves used to calculate the pKa values. The pKa values of stearic, elaidic, oleic, linoleic, and linolenic acids were found to be 10.15, 9.95, 9.85, 9.24, and 8.28, respectively. These pKa values were in the same order as area per molecule values of fatty acids in spread monolayers. This suggests that as area per molecule increases the intermolecular distance increases and pKa decreases due to reduced cooperation between adjacent carboxyl groups.