Abstract The synthesis of vinyl acetate from acetylene and acetic acid has been carried out using two industrial fluidized reactors. Both reactors have an inverted conical shape with a conical angle of 3°20′, and the diameter of the base is 3.28 m. Both have a production capacity of 50 ton/day. An activated carbon supported zinc acetate catalyst of 0.4 mm in average diameter is used. In order to investigate the optimum fluidizing and reaction conditions with the above industrial reactors, a series of operational tests have been carried out over a period of two years. As a result of such operation tests, it has been confirmed that the subject catalyst is free from hysteresis, because, in the operational tests using the catalysts with the same activity level, it shows the same reaction activity under the same reaction conditions, irrespective of the procedure for setting such reaction conditions. Further, it is considered that the rule of additivity of catalyst activity can be applied to this reaction system, because the subject catalyst shows a reaction activity corresponding to its mixing ratio, when using mixtures of catalysts with different activity levels. In these operational tests, we have introduced a method to calculate space time yield (STY) values at a certain past time point, under such conditions that STY changes on standing by a deterioration of catalyst activity. We measured the STY at a certain time point under certain reaction conditions and continued the operation under the same conditions. Several days later, we measured the STY under the same reaction conditions and, then, proceeded with the operational tests under different reaction conditions and measured the STY again. The STY under the latter reaction conditions at the first time point can be estimated by multiplying the ratio of STY actually measured, just before and after changing reaction conditions, and the STY value measured at the first time point.