  While the partition function for QCD in a magnetic field $H$ has been
calculated before within chiral perturbation theory up to two-loop order, our
investigation relies on an alternative representation for the Bose functions
which allows for a clear-cut expansion of thermodynamic quantities in the
chiral limit. We first focus on the pion-pion interaction in the pressure and
show that -- depending on magnetic field strength, temperature and pion mass --
it may be attractive or repulsive. We then analyze the thermodynamic properties
in the chiral limit and provide explicit two-loop representations for the
pressure in the weak magnetic field limit $|qH| \ll T^2$.
