The only known exceptions to the Mattauch isobar rule are the cases of antimony-123 and tellurium-123 and of hafnium-180 and tantalum-180m, where both nuclei are observationally stable. It is predicted that Te would undergo electron capture to form Sb, but this decay has not yet been observed; Ta should be able to undergo isomeric transition to Ta, beta decay to W, electron capture to Hf, or alpha decay to Lu, but none of these decay modes have been observed.
In addition, beta decay has been seen for neither curium-247 nor berkelium-247, though it is expected that the former should decay into the latter. Both nuclides are alpha-unstable.
As mentioned above, the Mattauch isobar rule cannot make predictions as to the half-lives of the beta-unstable isotopes. Hence there are a few cases where isobars of adjacent elements both occur primordially, as the half-life of the unstable isobar is over a billion years. This occurs for the following mass numbers:
* 40 (Ar and Ca stable; K unstable)
* 50 (Ti and Cr stable; V unstable)
* 87 (Sr stable; Rb unstable)
* 113 (In stable; Cd unstable)
* 115 (Sn stable; In unstable)
* 138 (Ba and Ce stable; La unstable)
* 176 (Yb and Hf stable; Lu unstable)
* 187 (Os stable; Re unstable)