Volume II/24 presents the spectroscopic data on diamagnetic and paramagnetic molecules as well as on molecular ions and radicals up to date considering the publications up to and partly including 1997. The spectroscopic information collected in this volume has been obtained principally from gas phase microwave measurements. Volume II/24 is a supplement to volumes II/4, II/6, II/14, and II/19 and is planned to appear in five subvolumes, the last of which will contain the substance index taking into account all molecules worked at since 1967 in the volumes mentioned and will provide cross references to and between all tables of these. The present subvolume II/24A contains the general introduction and tables of rotational and centrifugal distortion constants of diamagnetic diatomic, linear and symmetric-top molecules, i.e. data which roughly specifies the frequencies of the rotational transitions of these types of rotors.