  We present ISO LWS observations of the proto-planetary nebula CRL 618, a star
evolving very fast to the planetary nebula stage. In addition to the lines of
12CO, 13CO, HCN and HNC, we report on the detection of H2O and OH emission
together with the fine structure lines of [OI] at 63 and 145 um. The abundance
of the latter three species relative to 12CO are 4 10^{-2}, 8 10^{-4} and 4.5
(approximate value) in the regions where they are produced. We suggest that
O-bearing species other than CO are produced in the innermost region of the
circumstellar envelope. The UV photons from the central star photodissociate
most of the molecular species produced in the AGB phase and allow a chemistry
dominated by standard ion-neutral reactions. Not only allow these reactions the
formation of O-bearing species, but they also modify the abundances of C-rich
molecules like HCN and HNC for which we found an abundance ratio of roughly 1,
much lower than in AGB stars. The molecular abundances in the different regions
of the circumstellar envelope have been derived from radiative transfer models
and our knowledge of its physical structure.
