  We extend chiral perturbation theory to study a meson gas out of thermal
equilibrium. Assuming that the system is initially in equilibrium below the
critical temperature and working within the Schwinger-Keldysh contour
technique, we define consistently the time-dependent pion decay functions, the
counterparts of the pion decay constants, and calculate them up to next to
leading order. The link with curved space-time QFT allows to establish
nonequilibrium renormalisation. The short-time behaviour and the applicability
to a heavy-ion collision plasma are also discussed in this work.
