The Landolt-Börnstein volume II/30 contains data on the geometric parameters (internuclear distances, bond angles, dihedral angles of internal rotation, etc.) of free polyatomic molecules including free radicals and molecular ions. This volume is made of two subvolumes, each containing the geometric parameters determined in the gas phase either by an analysis of the rotational constants (and sometimes the vibrational constants) obtained from microwave, infrared, Raman, electronic, and photoelectron spectroscopy or by an analysis of electron diffraction intensities. All data obtained by these methods have been critically evaluated and compiled. The data are presented separately for each molecule, together with original references and computer-drawn figure(s). In this first subvolume, II/30A, the structures of inorganic molecules and molecules containing one or two carbon atoms are listed. Subvolume II/30B covers molecules with three or more than three carbon atoms.