The pK(a) values of ten phenolic compounds (phenol, 4-fluorophenol, 4-iodophenol, 3-bromophenol, 4-methylphenol, 4-nitrophenol, 4-ethylphenol, 2-methylphenol, 3-nitrophenol, and 3-methylphenol) in 10% (v/v) acetonitrile-water mixture using spectrometric measurements have been determined. The electronic absorption spectra of phenolic compounds at various pH values (pH 2.0 - 11.0) at 200-500 nm intervals were recorded. The calibration of the electrode system was done potentiometrically by Gran's method. Data were processed using the programme STAR (Stability Constants by Absorbance Readings). The obtained results are in good concordance with literature values.