  We use chiral perturbation theory for SU(2) to compute the leading
corrections to the thermal mass of the pions and the pion decay constant in the
presence of the magnetic field in a low-temperature expansion. The magnetic
field gives rise to a splitting between $M_{\pi^0}$ and $M_{\pi^{\pm}}$ as well
as $F_{\pi^0}$ and $F_{\pi^{\pm}}$. We also calculate the free energy and the
quark condensate to next-to- leading order. The results suggest that the
critical temperature $T_c$ for the chiral transition is larger in the presence
of a constant magnetic field, in agreement with most model calculations but in
disagreement with recent lattice calculations.
