A more detailed investigation of the influence of heavy water on the two main kinetic pathways underlying the mechanism of the Bray-Liebhafsky (BL) reaction is performed. The course of the BL reaction at four temperatures was monitored with an improved experimental setup, enabling simultaneous recordings of the potential of the platinum electrode, the concentration of I-2, and the oxygen production rate. It is confirmed that by replacing ordinary water with heavy water, the oxidation pathway is more effectively accelerated than the reduction pathway. Influence of the isotopic substitution on the acidity of the BL system, VIS spectra of iodine in D2O, and the reaction between iodine and deuterated peroxide are investigated in separate experiments. It is concluded that the observed enhancement of the oxidation pathway cannot be simply explained by the isotopic effect. Hence, the possible role of bulk water in the process of shifting the whole reaction mechanism toward the oxidation pathway is discussed.