Natural yttrium (39Y) is composed of a single isotope yttrium-89.
The most stable radioisotopes are 88Y, which has a half-life of 106.6 days, and 91Y, with a half-life of 58.51 days.
The dominant decay mode below the stable 89Y is electron capture and the dominant mode after it is beta emission.
Thirty-five unstable isotopes have been characterized.
90Y exists in equilibrium with its parent isotope strontium-90, which is a product of nuclear fission.