Ti-Cr-V alloys are known to absorb protium (hydrogen atom) up to H/M = 2. However, the Cr-rich alloys, e.g. Ti-Cr-20 at%V alloys containing more than 56 at%Cr, absorb up to H/M = 1 because of the formation of the mono-protides (mono-hydrides). The appearance region of the mono-dueteride was found to be more Cr-rich compositions than that of the mono-protides. For example, Ti-Cr-20 at%V alloys containing more than 60 at%Cr absorb deuterium up to D/M = 1. Therefore, Ti-56 at%Cr-20 at%V alloy absorbs protium up to H/M = 1, but deuterium up to D/M = 2. As higher protium desorption capacity is achieved by increasing the Cr content in the region of the di-protide, there is some possibility of increasing the protium desorption capacity in Ti-Cr-V alloys using the isotope effects on absorption-desorption properties of the Ti-56 at%Cr-20 at%V alloy. This paper aims to clarify the isotope effects on protium and deuterium absorption properties in the Ti-56 at%Cr-20 at%V alloy. It was found that the memory effect for the absorption plateau pressure appears only once when deuterium was absorbed after desorbing protium, and the memory effect for the absorption capacity appears when protium was absorbed after desorbing deuterium. The protium desorption capacity after deuterium treatment, namely, after desorbing deuterium showed twice as high as that without deuterium treatment in the Ti-56 at%Cr-20 at%V alloy. Increasing protium desorption capacity for deuterium treatment was caused by increasing di-protide formation and decreasing mono-protide formation.