  We present preliminary results on a study about the thermal variation of the
charged and neutral pion masses to one loop, analyzing their electromagnetic
difference, in the context of Chiral Perturbation Theory with two flavours, as
well as using a light resonance model. We find that the pion mass difference
increases for, at least, low and intermediate temperatures, unlike the chiral
limit decreasing result. The axial-vector mixing arising from chiral
restoration smooths the Debye-screening temperature increase. Taking into
account further corrections due to axial and vector resonances, dominated by a1
and rho particles respectively, does not change significantly the ChPT
prediction.
