Abstract: In order to explore the effect of Ti-Mo interdiffusion on metal hydrogen absorption, hydrogen absorption isotope effects at the TiMo alloy due to the interdiffusion at interface were investigated by using ion beam analysis (IBA) which is a nondestructive testing method. A Ti film of a few microns was deposited on a Mo substrate using a vacuum evaporation coating method, and interdiffusion layers appeared at the TiMo interface. The results indicate that there is a single gradient of elemental Ti or Mo concentration across the interdiffusion layer. Physical phase analyses show that Mo is substituted for Ti and exists as a solid solution. The effect of surface carbon (C) and oxygen (O) impurities on hydrogen (H) and deuterium (D) absorption was reduced by the argon ion sputtering thinning method. The presence of O below the detection limit and a small amount of C on the surface of the sample after sputter thinning have a minimal effect on hydrogen absorption. At the same time, the interdiffusion interface is brought closer to the surface. Hydrogen absorption experiments were carried out in an ultrahigh vacuum device. Both single and mixed absorption experiments for H2 and D2 were conducted, and the samples were analyzed by IBA. The results show that for the samples with clean surfaces, the distribution of hydrogen and deuterium in the solid phase decreases with the increase of molybdenum concentration after absorption of either hydrogen or deuterium gas. This is because that the presence of Mo reduces the bonding ability between the Ti atoms and interstitial H/D atoms, resulting in an increase in the equilibrium pressure of hydrogen/deuterium absorption and a decreased amount of hydrogen absorption. Moreover, the hydrogen concentration tends to decrease more slowly than the deuterium concentration with the increase of depth (i.e. the increase of Mo concentration). When Mo concentration in Ti is greater than 10%, the decrease in lattice constant leads to the reduction of lattice clearance volume. So hydrogen with smaller radius is more likely to occupy the volume gap position, resulting in a sharp drop in the content of largerradius deuterium atoms in the region with high Mo concentration. In mixed hydrogen absorption experiments, the pressure ratio of deuterium to hydrogen (p(H2)∶p(D2)) decreases with the increase of Mo content for p(H2)∶p(D2)≥0.5∶1, while for p(H2)∶p(D2)<0.5∶1, the ratio increases with the increase of Mo concentration. In summary, the TiMo alloy interdiffusion interface exhibits obvious isotope effects, i.e., hydrogen is more readily absorbed than deuterium. And the presence of Mo is detrimental to the absorption of hydrogen isotope gases in the system. This study not only provides new ideas for hydrogen isotope analysis in metals, but also has important implications for further understanding of matrix atomic diffusion behavior, hydrogen storage materials and improving the performance of metal hydride target materials in use.