Generally categorized in a large family of transition metal dichalcogenide layer crystals, molybdenum disulfide (MoS2) has been the subject of significant research interest for applications in non-aqueous lithium batteries [1], catalytic hydrodesulphurization of petroleum [2], and antiwear achievement by reduction in friction [3-5]. The successful application of this semiconductor compound originates largely from the sandwich interlayer structure, loosely bound by Van der Waals forces, evidenced by easy cleavage in the [0 0 1] direction along which the (S-Me-S) layers are stacked to form the crystal [6]. Apart from its natural state, known as molybdenite, crystalline MoS2 can also be synthesized by hightemperature solid state reaction between stoichiometrically mixed molybdenum and sulphur powders in vacuum [7-9], thermal decomposition of ammonium tetrathiomolybdate ((NH4)2MoS4) [10, 11] or amorphous molybdenum trisulfide (MoS3) [9, 12], and by thermally assisted transformation of amorphous MoS2 powders [13]. Synthetic MoS2 can also be prepared by sputtering [14], electrodeposition [15] and chemical deposition techniques [16]. The present letter reports formation of MoS2, by appropriate thermal treatment of precipitates resulting from acidification of ammonium tetrathiomolybdate solution. As mentioned previously, although MoS2 can be prepared from tetrathiomolybdate by thermal decomposition, the current work is of practical interest in that it produces evidence for in situ formation of MoS2 during anodizing of aluminium and its alloys in thiomolybdate electrolyte. Compared with the extensive research on the formation of alumina by anodizing, the formation of MoS2 during anodization has been rarely reported. Skeldon and Thompson [17] recently proposed to overcome the general poor surface retention of MoS2 by incorporating it, by anodizing, into a pre-anodized, hard porous alumina film on aluminium for tribological applications. The main anodic reaction during anodizing of aluminium results in proton generation at the pore base/electrolyte interface as follows