Biochemical characteristics of polyphenol oxidase (PPO) from Narince grapes were studied in two consecutive years (2006 and 2007). The optimum pH and temperature of the two PPOs were found to be same (5.49 and 45 A degrees C). The apparent substrate specificities were established from V-max/K-m as follows: caffeic acid > 4-methylcatechol > catechol > pyrogallol for two enzymes. There were marked differences between the two enzymes in terms of thermal stability. The effects of inhibitors varied in a dose-dependent manner. The degree of inhibition exerted by the inhibitors tested varied according to the harvest year.