There are hardly any methods for the determination of eugenol other than the measurement of ultraviolet absorption and nonaqueous titration. Eugenol possesses a substituent in the position para to phenolic hydroxyl and structurally, it appears to be negative to indophenol reaction. Following the previous works on Colorimetric determination of pharmaceutics by the use of indophenol reaction, eugenol was found to be positive to the indophenol reaction. It has already been found that, when the substituent in the para-position of phenolic hydroxyl is easily liberated, the compounds show positive indophenol reaction. It may, therefore, be assumed that, in the case of eugenol, the allyl group in the para-position is easily liberated and the compound shows positive indophenol reaction. Satisfactory results were obtained by colorimetric determination of eugenol and also that in clove oil by application of 0.05% dimethyl-p-phenylenediamine hydrochloride and 0.02% of sodium hypochlorite to eugenol dissolved in Sörensen phosphate buffer of pH 8.0, extraction of the indophenol pigment thereby formed with isobutanol, and measurement of absor-bancy of the extract solution at 610mμ.