The hydrogen isotopic exchange with basic and acidic agents has been widely investigated with the aim of preparing fully deuterated biphenylene. It has been noted that the very strong base KOD completely decomposes the hydrocarbon, while the strong base NaOD leads to random exchange, and the medium strength base Ca(OD)2gives preferential exchange in α-position. Since the stability of biphenylene is less in acid mixtures than in basic ones, only acids of medium strength, which give preferential exchange in β-position, can be used.Good results in preparing fully deuterated biphenylene are obtained from two methods. The first of these consists of two exchanges which occur successively; one with an acid mixture and the other with a base. Both agents are of medium strength. The second method entails repeated exchanges with the strong base NaOD supported on Ca(OD)2. The latter method gives the highest isotopic purity and the best yield, as well as being much easier to carry out.