This is subvolume C of the Landolt-Bornstein Volume II/29 "Molecular Constants Mostly from Microwave, Molecular Beam, and Sub-Doppler Laser Spectroscopy," which is planned to appear as a series A, B, C and D1 and D2 for the diamagnetic, and E for the paramagnetic linear and polyatomic species, respectively. Like in the preceding volumes II/19 and II/24, which have appeared in 1992 and 1999, the diamagnetic substances are arranged in the manner suggested by Hill ("Hill's system," 1900), meaning an almost strict alphabetical order; details are given in the General Introduction on the following...