The threshold energy of the Mg-26(p, n)Al-26(m) reaction has been meas
ured to be 5209.46+/-0.12 keV, using a 1 V standard as reference, and
taking account of effects due to finite beam energy spread, nonuniform
proton energy loss in the target, and ionization of the magnesium ato
ms. The Q value derived for the Al-26(m) beta decay then becomes 4232.
17+/-0.12 keV, which is in clear disagreement with the most precise of
the existing measurements. A further measurement is proposed.