For everyone concerned with the technology and application of metals and alloys and with the development of new metallic materials, a detailed knowledge of phase equilibria is indispensable. Also, information on the thermodynamical and crystallographical data of the systems under investigation is essential, and often metastable crystalline phases as well as quasicrystalline or amorphous alloys are of interest. Vol. IV/5 therefore presents all these data. Because of the large amount available of experimental evidence and thermochemical calculations, a presentation in one volume, as it was realized several decades ago in the widely used book of M. Hansen 'Aufbau der Zweistoff-Legierungen' proved to be impossible. So volume IV/5 had to be divided into several subvolumes which cover - in alphabetical order - all binary systems of importance. Subvolume IV/5a, the first of the series, deals with the systems Ac-Au ... Au-Zr. Further subvolumes will follow shortly.