Nano scale molybdenum disulfide (MoS2) is a very promising material for the next generation electronics and energy storage devices. The active defect sites located at the periphery of the lattice may influence certain redox reactions, thus classifying it as an excellent non-noble metal catalyst. One effective approach to synthesize large scale MoS2 films is through simple thermolysis of (NH4)(2)MoS4 precursor. Herein, a combined in situ X-ray and Ultra-Violet photoelectron spectroscopies (XPS/UPS) study has been carried out to follow the evolution of atomic composition, the surface chemical species and the electronic properties (Work Function and Ionization Potential) during the thermal decomposition of (NH4)(2)MoS4 salt in Ultra High Vacuum conditions. It was found that, up to 400C, thermal conversion of (NH4)(2)MoS4 salt to transient intermediates, across amorphous MoSx phase have been performed towards to MoS2 out of significant percent of suphur active sites. The active sites have been decreased upon heating at higher temperatures. Assignment of the thermally evolved active species at defect sites and the correlation of surface atomic concentration combined with either the Ionization Potential or the Work Function of annealed MoSx nanoflakes makes this study a knowledgeable factor of awareness through electrocatalytic properties of MoS2-based nanostructures.