Volume II/19 is a supplement to the previous published volumes II/4, II/6 and II/14. The slight change in the title of the series reflects current trends in the applications of spectroscopic methods. The four volumes II/4, II/6, II/14 and II/19 together comprise a comprehensive collection of critically evaluated data on constants of free molecules obtained by classical and modern spectroscopy which have appeared in the literature until 1989/1990. The present subvolume 19d1 provides critically evaluated data which have appeared in the literature up to 1991/92. Subvolume II/19d3 provides a substance index for all volumes II/4, II/6, II/14 and II/19.