Density and thermal corrections to the mass of the pions are studied in the framework of the SU(2) low energy effective chiral lagrangian, in terms of the isospin chemical potential μ_I. We concentrate the discussion in the region where the isospin chemical potential (absolute value) becomes bigger than the pion mass at zero temperature and density, i.e. in the phase where the condensed π^- phase appears (for negative chemical potential). We are able to calculate the thermal and density

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... n of masses in the limits where |μ_I|≫ m and where |μ_I| ≳ m. We also identified the phase transition curve.