This list is no longer maintained. Please refer to McGuire+, 2018ApJS Vol239, p17 periodically updated (e.g. arXiv:2109.13848) McGuire, 2021 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules.

The reported interstellar and circumstellar molecules.

NRAO National Radio Astronomy Observatory

A GSFC List including Ices.

Resources for Frequencies

Splatalogue .

NIST Recommended Rest Frequencies for Observed Interstellar Molecular Microwave Transitions by Frank J. Lovas

Molecular Spectroscopy Home Page (Includes transitions not yet observed in the ISM) by Herb Pickett

The Cologne Database for Molecular Spectroscopy additions.

The Astrochemistry Working Group of the International Astronomical Union home page lists other resources