Abstract: The experimental results showed that H₃PW₁₂O₄ and its salts are effective catalysts for oxidative dehydrogenation of ethylbenzene to styrene. The selectivity was as high as 98% at 18% conversion.Their activities were nearly the same regardless of their differences in acid strength and acidity. The most important feature of this reaction system appeared that the catalytic activity increases with reaction time in the beginning period, the "Break-in"period.During the "Break-in" the H/C ratio of coke decreases, while the amount of coke formed on the catalyst surface increases. some quinoid compounds were detected by thin layer chromatography and mass spectrometry and they are considered to be the active sites of tbe oxidative dehydrogenation of ethylbenzene.