Chemistry is a subject experiencing very rapid growth. New areas of researeh are opened and new methods are developed with a speed whieh must seem impressive. The pieture has, however, another side represented by the great number of unsolved or poorly solved problems left behind when a branch of chemistry is no longer fashionable. When three years ago I beg;m to plan this book I felt that the molecular adduets of oxo­ compounos constituted just sueh a field of research, and that they might acquire greater interest if approached from the standpoint of modern chemistry. The developments of the last three years have been such, however, that it can no longer be said that interest is lacking. From that point of vic~w a book on this subject may now seem unnecessary. On the other hand individual contributions to this field have been very scattered, and the essential problems have not always been recognized. It is hoped therefore tlat this book will help to coordinate work on these compounds and that future investigations will derive some benefit as a result. For this purpose a systematie treatment is needed and the first and larger part of the book is an attempt to present a fairly complete review of past experimental work. The treatment is not detailed; it is aimed instead at giving the reader a chance to find all the pertinent references.