  The pionium decay width is evaluated in the framework of chiral perturbation
theory and the relativistic bound state formalism of constraint theory.
Corrections of order O(\alpha) are calculated with respect to the conventional
lowest-order formula, in which the strong interaction amplitude has been
calculated to two-loop order with charged pion masses. Strong interaction
corrections from second-order perturbation theory of the bound state wave
equation are found to be of the order of 0.4%. Electromagnetic radiative
corrections, due to pion-photon interactions, are estimated to be of the order
of -0.1%. Electromagnetic mass shift insertions in internal propagators produce
a correction of the order of 0.3%. The correction due to the passage from the
strong interaction scattering amplitude evaluated with the mass parameter fixed
at the charged pion mass to the amplitude evaluated with the mass parameter
fixed at the neutral pion mass is found to be of the order of 6.4%.
