Naturally occurring erbium (₆₈Er) is composed of six stable isotopes, with ¹⁶⁶Er being the most abundant (33.503% natural abundance). Radioisotopes have been characterized with from ¹⁴⁵Er to ¹⁷⁵Er, all having half-lives less than ten days: the most stable are ¹⁶⁹Er (9.39 days), ¹⁷²Er (49.3 hours), and ¹⁶⁰Er (28.58 hours). All of the remaining radioactive isotopes have half-lives that are less than 11 hours, and the majority of these have half-lives that are less than 4 minutes. This element also has numerous meta states, with the most stable being ^149m1Er (t_1/2 = 8.9 seconds).

The primary decay mode before the most abundant stable isotope, ¹⁶⁶Er, is electron capture to holmium isotopes, and the primary mode after is beta decay to thulium isotopes. All isotopes of erbium are either radioactive or observationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed.

Daughter products other than erbium