ISO observations of gas and dust chemistry in star-forming regions

Recent results of searches for absorption lines of gas-phase molecules and solid-state species in the ISO spectra of embedded massive young stars are reviewed. The ISO-SWS is very well suited for the study of interstellar ices and provides an unbiased overview of their composition for the first time. Early highlights include observations of abundant, widespread solid CO2, and the detection of solid CH4 and HCOOH. Possible candidates for other weak features are discussed. In the gas-phase, ISO provides important information on the abundance of H2O in warm regions, through observations of the vibration-rotation lines with the SWS and pure rotation lines with the LWS. Gas-phase CO2 is also detected, but with a surprisingly low abundance. Recent data on gas-phase CH4, C2H2 and HCN are presented as well. Gas/solid state ratios are determined, which give information on the importance of gas-grain chemistry, and on the evolutionary state of the object.