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 ^ { 12 } CO , ^ { 13 } CO , HCN and HNC , we report on the detection of H _ { 2 } O and OH emission together with the fine structure lines of [ OI ] at 63 and 145 \mu m. The abundance of the latter three species relative to ^ { 12 } CO 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 \simeq 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 .

Subject headings : infrared : stars — ISM : molecules — line : identifications — planetary nebulae : individual ( CRL618 ) — stars : abundances — stars : carbon