Pionium Decays and Pion Polarizability

We calculate the decay of pionium atoms into two photons. The pion polariz-ability gives rise to a 10% correction to the corresponding decay width for pointlike pions. This opens the possibility to obtain the polarizability of the charged pion from a future measurement of this decay width if the ππ-scattering lengths are known to better than 5% precision. We also comment on the contribution of the axial anomaly to the decay of pionium into γπ 0 .