In our previous study, we showed that the anti-disproportionation properties of Zr_0.8Ti_0.2-xNb_xCo alloys were remarkably improved by the co-substitution of Zr with Ti and Nb. However, the practical application of these alloys in handling of hydrogen isotopes necessitates the first hand knowledge of hydrogen isotope effect. Herein, we discuss the hydrogen isotope effect on storage properties of Zr_0.8Ti_0.2-xNb_xCo alloys. According to PCT measurements on desorption of deuterium from the Zr_0.8Ti_0.2-xNb_xCo deuterides and comparison with corresponding hydrides, the deuterides require relatively lower temperature to achieve the desired equilibrium pressure. DSC measurements reveal a significant decrease in the activation energy for hydrogen/deuterium desorption reactions when Zr is substituted with Ti and Nb. Furthermore, it is observed that the activation energy of deuterium desorption is lower than the desorption of hydrogen from analogous hydride. Isotope effect on isothermal disproportion studies on Zr_0.8Ti_0.2-xNb_xCo-deuterides divulge that Zr_0.8Ti_0.2-xNb_xCo-deuterides have superior anti-disproportionation properties over corresponding hydrides, and further improvement is anticipated for the Zr_0.8Ti_0.2-xNb_xCo-tritides. This study revealed the significant impact of Ti and Nb co-substitution on hydrogen isotope storage properties of Zr_0.8Ti_0.2-xNb_xCo alloys, making them potential candidates for handling hydrogen isotopes.