Polyphenol oxidase catalysing the oxidation of 4-methylcatechol in reverse micelles of AOT/cyclohexane had an optimum temperature 15 degreesC higher than in aqueous medium. However, the enzyme lost stability when it was preincubated in reverse micelles in the absence of substrate regardless of the temperature although the effect was more pronounced at higher temperatures. The thermostability of polyphenol oxidase is higher when it is injected in reverse micelles containing buffer than when injected in initially empty micelles. Moreover, the thermostability of polyphenol oxidase in reverse micelles is strongly dependent on the size of the micelles, the bigger the micelle the greater the stability. The thermoinactivation of the enzyme follows a monomolecular process characteristic of a conformational change so the protein is protected by ligands towards inactivation, p-Nitrophenol as competitive inhibitor and acetyl tyrosine ethyl eater as alternate substrate increase the half-life of the polyphenol oxidase by about 2.5 and 4 times respectively. This finding may allow the use of the enzyme at higher temperatures with a gain in its stability.