The present volume continues the edition of a number of supplement volumes dealing with the elements tungsten and molybdenum. The compounds of molybdenum with noble gases, hydrogen and oxygen, anhydrous antimo- ny-, bismuth- and alkalimolybdates as well as compounds of molybdenum oxides with oxides of other metals have been described in volume B 1 and B 2. The oxide hydrates and the molybdate ions are dealt with in volume B 3a and B 3b. The volume molybdenum supplement B 4 contains the hydrous oxocompounds of the metals Sb to Cr with molybdenum. Description of the element molybdenum is covered by the supplement volumes A 1, A 2a, A 2b and A 3. In the first part of the volume the nitrogen-containing compounds are described. The Mo-N system shows Mo2N and MoN as the stable nitrides. Molybdenum metal dissolves nitrogen to some extent but only at high temperatures. The second part contains the description of the compounds of molybdenum with fluorine. Studies of their physical and chemical properties are complicated by the high sensitivity to traces of water resulting in the formation of oxide fluorides. These and the oxofluoromolybdate ions are discussed in separate sections. In the last part the molybdenum chlorides MoCln with n = 1 to 6 are described. The molybdenum chlorine compounds containing additional elements will be described in the following volume.