title: Chemical evolution of circumstellar matter around young stellar objects.
abstract: Recent observational studies of the chemical composition of circumstellar matter around both high- and low-mass young stellar objects are reviewed.  The molecular abundances are found to be a strong function of evolutionary state, but not of system mass or luminosity.  The data are discussed with reference to recent theoretical models.
authors.affiliation: Leiden Observatory, The Netherlands.
authors.forename: E F
authors.initials: EF
authors.lastname: van Dishoeck
authors.forename: G A
authors.initials: GA
authors.lastname: Blake
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D055598:Chemical Phenomena; D002621:Chemistry; D004391:Dust / Q000032:analysis*; D004602:Elements; D005118:Extraterrestrial Environment; D005740:Gases / Q000032:analysis*; D006838:Hydrocarbons / Q000737:chemistry; D008956:Models, Chemical
chemical_list: D004391:Dust; D004602:Elements; D005740:Gases; D006838:Hydrocarbons