We propose high spectral resolution observations of high level CO
 emission lines in a sample of evolved stars, in order to study the
 evolution of the winds and circumstellar envelopes from the AGB to the
 Planetary Nebula phases. We will determine the thermal structure of
 the circumstellar gas shells, as well as the mass-loss rates during
 the superwind phase. The sample of objects chosen for the study
 includes AGB stars with low, intermediate and very high mass-loss
 rates, post-AGB stars with low, intermediate and high central star
 temperatures, and low- and high-excitation PN. Both O-rich and C-rich
 examples of each type are included. The CO observations will be
 modelled using a full radiative transfer code in the comoving frame in
 the case of AGB stars and the coolest post-AGB stars, whilst for the
 post-AGB stars and PN we will use modified codes originally written
 for the study of photodissociation regions which we have already used
 succesfully to model KAO observations of CO in a few of the brightest
 of such sources. The results will allow us to follow the evolution of
 physical conditions (gas temperature and density) in the hottest,
 densest parts of the circumstellar envelopes, close to the central
 stars.