Volume II/20 provides critically evaluated data on free molecules, obtained from infrared spectroscopy and related experimental and theoretical investigations. The volume is divided into four subvolumes, Molecules, B: Linear Triatomic Molecules, C: Nonlinear Triatomic Molecules, D: Polyatomic Molecules. The present subvolume II/20 B2° contains data for 16O12C16O. Subvolume II/20 B2ß containing data for CO2 isotopomer and Subvolume B3 containing data for CS2 ... N3, and further subvolumes are in preparation.