The influence of heavy water on the Bray-Liebhafsky (BL) oscillating reaction was investigated for varying amounts of D2O, at three different temperatures. Evolution of the system was monitored potentiometrically. Simultaneous recordings of gaseous oxygen over the reaction solution were also performed. In separate experiments the iodine concentration was monitored spectrophotometrically. Replacement of H2O by D2O progressively intensifies the reactions of oxidation of the iodine species, compared to the reactions of their reduction. As a result there is a critical ratio of D2O/H2O, after which the dynamics of the system is considerably altered. The same effect is observed at all temperatures, but it is more pronounced at lower temperatures. Two possible explanations for the progressively intensified oxidation are discussed : the reverse isotope effect and the selective energy transfer. It is established that the oxidation and reduction of the iodine species in the BL reaction do not proceed through the same intermediates. The important role of bulk water in surmounting the high activation energy threshold of the oxidation branch is revealed in the experiments.