Decomposition of ozone into hydroxyl radicals is one of the most frequently discussed hypotheses of the heterogeneous catalytic ozonation mechanism. Estimation of catalytic activity of solid catalysts in the reaction requires the reference system (without the catalyst) with the same pH in which spontaneous ozone decomposition will occur. However, the contaminants of many ozonation catalysts may significantly influence the process of ozone decomposition just by changing pH of water. In this paper the effect of active carbon contaminants on the process of ozone decomposition is discussed. Contaminants are mainly of alkaline character and moreover are easily dissolving in water or acids. Thus the lack of precise control of pH leads to an observation of false catalytic effects connected with the change of pH after the catalyst introduction into the aqueous ozone solution. To differentiate between the effects caused by increased pH and true catalytic activity of potential catalysts strict control of pH is advised. This paper shows that requirement of the same pH in the catalytic and reference system is not as simple as thought before. The observation of the contaminants effects on the process of ozone decomposition explains at least some controversial literature results. (c) 2013 Elsevier B.V. All rights reserved.