The relative role of the stellar radiation field, the stellar outflows and the interstellar radiation field (ISRF) in transforming the molecular ejecta into atomic gas was the subject of our ISO LWS and SWS spectroscopy study of 24 evolved stars which span the range from AGB stars to proto-planetary nebulae (PPNs) and PNs. The far-infrared (FIR) atomic fine-structure lines are powerful probes of the warm atomic gas in photodissociation regions (PDRs) and shocks. This paper summarizes and compares the ISO spectroscopy studies of carbon-rich (C-rich) and oxygen-rich (O-rich) evolved stars, published by Fong et al. (2001) and Castro-Carrizo et al. (2001), respectively. We find that photodissociation, not shocks, is responsible for the chemical change from molecular to atomic gas.