  We analyze the onset of classical behaviour in a scalar field after a
continuous phase transition, in which the system-field, the long wavelength
order parameter of the model, interacts with an environment of its own
short-wavelength modes. We compute the decoherence time for the system-field
modes from the master equation and compare it with the other time scales of the
model. Within our approximations the decoherence time is in general the
smallest dynamical time scale. Demanding diagonalisation of the decoherence
functional produces identical results. The inclusion of other environmental
fields makes diagonalisation occur even earlier.
