Abstract: The physiological causes of the core browning of pear fruit during storage were studied. The polyphenol oxidase activity in the core, flesh and skin were determined and the site of highest aetivity was found in the core. The distribution of the total polyphenolic content in the different parts of the pear fruit was similar to that of polyphenol oxidase activity. In the brown core the amount of the polyphenolic substances decreased markedly as compare with the normal ones The separation and identification of the polyphenolic substances were carried out by paper chromatography and high voltage electrophoresis. Seven compounds were found in: the core; five in the flesh and six in the skin. The chlorogenic acid was the major component. The ultraviolet spectra of the chlorogenic acid oxidized by the polyphenol oxidase indicated: that the absorption peak of chlorogenic acid at 324 mμ decreased after the oxidation. The absorption spectra gradually increased toward the utraviolet region, and was similar to that of the natural brown product of pear fruit. These results show that the physiological cause of the core browning is due to the enzymic browning reaction and the principal substate in the core is chlorogenic acid.