Since the publication of Volumes II/7 in 1976 and its supplements II/15 in 1987 and II/21 in 1992, the information on the structure of free molecules in the ground state and in excited electronic states has increased considerably. Therefore this volume II/23 contains data from 148 inorganic and 498 organic polyatomic free molecules (including free radicals and molecular ions) published between 1990 and 1993 inclusively and a small number of structures...