Ionization constants of phenol, p-cresol and orthochlorophenol have been determined at varying temperature from 25 to 50 degrees C and in 10, 20, 30 and 40 percent methanol-water solvent system at 25 degrees C by potentiometry. pK(a) values of all these compounds also been determined in varying percentage of methanol-water solvent system by liquid chromatography on a reversed phase column at 25 degrees C The purpose of this investigation was to obtain the data for the dissociation of phenol derivative using diverse methods with particular emphasis on the effect of chloro (-C1) and methyl (-CH3) group on phenyl ring which in turn effect on pK(a) values. Results obtained by both methods were compared and effect of substituents on phenyl ring is being discussed in light of Hammets equations. Potentiometric data was analysed by computer program written in GW-BASIC for calculation of pK values.