Planetary nebulae (PNe) are believed to form when the envelope of gas and dust, ejected at the end of the AGB phase of stellar evolution, is heated by the hot stellar nucleus, left after mass loss due to the “superwind” is terminated. The presence of residual molecular gas in PNe should therefore be important for understanding the formation and subsequent evolution of the nebulae. Observations (Bachiller et al. 1988) of CO line radiation in proto-planetary nebulae (PPNe), the transition phase from AGB stars to PNe, allowed to set very low upper limits on the molecular gas mass and to predict almost complete dissociation of the molecular envelope at the end of the PPN stage. On the other hand, observations (Huggins & Healy 1989; Healy & Huggins 1990) of CO lines detected intense signals from several PNe, including some evolved sources.