The pK(a) values of 6-semicarbazone 9-17 monomethyl substituted octadecanoic acids were determined using the potentiometric titration method. 2-Propanol was used as the non-aqueous medium and tetrabutylammonium hydroxide (TBAH) as the titrant in the experiments. The contribution of semicarbazone and methyl groups to the pK(a) values of octadecanoic acids was investigated and evaluated. In addition, observations were made on the IR spectra of the compounds.