In the course of my recent revision of the genus Rhododendron in Malaysia it has become evident, that the subsect. Irrorata, regarded as purely South Eastern Asiatic up to now, is also represented in the Malay Peninsula and Sumatra, whilst the subsect. Euvireya, outside of its large Malaysian area, has outposts in Indochina, and the subsect. Pseudovireya covers an enormous area from the Eastern Himalaya and S.W. China over the whole of Malaysia to New Guinea. Through the ’Species of Rhododendron’ (1930, 2nd ed. 1917), the Eastern Asiatic Rhododendrons are rather well known for India, Burma, China and Japan, but less so for Siam, while those of Indochina were omitted. It seemed therefore necessary to elaborate the latter critically, not only to fill the gap between our knowledge of the proper Eastern Asiatic and Malaysian Rhododendrons taxonomically, but also for a better understanding of the geographical relations within the old-world groups of the genus, i.e. their mutual overlapping in the area between the two supposed evolutionary centres of Rhododendron, the primary one in proper continental Asia, a secondary one in Malaysia. Various trips to Great Britain and France enabled me to study the material deposited in the herbaria of London (Brit. Museum Natural History), Kew and Edinburgh, and the specimens which Dop has had for his treatment of the genus in the Fl. Gén. de l’Indo-Chine (1930) in the Paris Museum. The more recently collected and not yet classified material at Paris helped us to enlarge our knowledge of the Indochinese Rhododendrons considerably. The type specimen of R. ningyuenense Hand.-Mazz. was kindly lent from the Botanical Institute of the Vienna University.