The acidity constants of four phenol derivatives, 4-methyl-2,6-bis(hydroxymethyl)phenol, 4-fluoro-2,6-bis(hydroxymethyl)phenol, 4-chloro-2,6-bis(hydroxymethyl)phenol, and 4-bromo-2,6-bis(hydroxymethyl)phenol, were determined using potentiometric and spectrophotometric methods at 0.10 M tetraethylammonium bromide (TEAB) as supporting electrolyte in different ratios of methanol + water mixtures. In the spectrophotometric method, only the acidity constant of the hydroxy group that directly attached to the aromatic ring (phenolic group) was determined, whereas, by the potentiometric method, the acidity constants of all three hydroxy groups could be determined. The results show that the acidity constants K-a1 decrease as the percent of methanol in the solvent mixture increases, but K-a2 and K-a3 do not show similar behavior. Thermodynamic parameters of these compounds in x(1) = 0.2 for methanol (1) + water (2) were determined in the temperature range (15 to 55) degreesC.