Abstract: Monitoring acidity at the ppm level in phenol-acetone-cumene process streams on a continuous basis is accomplished with a standard hydrogen electrode and a double-junction reference electrode with an external body solution comprising phenol, acetone, water, and a tetraalkyl- or tetraarylammonium salt, which provide a time invariant voltage without the necessity of adding any external solvent to the process stream. Phenol/acetone ratios can be varied from 1:10 to 10:1 which encompass the 1:1 ratio produced by the phenol-acetone process. A direct correlation of the electrode potential with the acidity of the medium is obtained. With the aqueous mixture isolated by a low flow ceramic junction from the process stream, no contamination of the process stream occurs.