Circumstellar CO emission has been detected in a number of featureless or weak SiC emission, low color temperature IRAS sources. The CO detections confirm the suggestion that these are either extreme carbon stars, carbon-rich proto-planetary nebulae (PPNs), or carbon-rich planetary nebulae (PNs). We find that the CO emission is relatively stronger for a given luminosity in post-AGB stars than AGB stars, suggesting a more efficient excitation mechanism is at work in the post-AGB phase. One probable post-AGB star was detected in HCN for the first time. The available HCN data suggest a rapid decline in HCN emission after the AGB. Molecular emission is shown to be a useful tracer of the evolution from asymptotic giant branch to the planetary nebula phase.