Transition metal dichalcogenides have a laminar structure, with strongly covalently bonded layers weakly interacting through van der Waals forces. They are of special interest also because of their unique properties once exfoliated in nanoflakes. We analyse the microstructure of oxidised TiS 2 nanoflakes with atomically resolved scanning transmission electron microscopy and propose a comprehensive model for their reactivity by means of first principles simulations. In particular we find that reaction to water proceeds from the edges of the flake, while it is thermodynamically possible but kinetically hindered in the middle, unless it is initiated by the presence of a surface vacancy. Importantly O substitution for S allows fine-tuning control of the flake bandgap, paving the way for the use of TiS 2-x O x alloys as surface catalysts and photovoltaic materials. . High-resolution image of a TiS 2 flake edge showing an extended crystalline region in the upper part. The bright dots in the image correspond to Ti atoms surrounded by S atoms. Below the dotted line an area of amorphous TiO2 is indicated. In the magnified HAADF image (blue box in the image) the Ti and S atom columns are well separated. Beside the HAADF image we present the corresponding Electron Energy Loss (EEL) spectrum images (elemental maps) of S, C, Ti and O respectively (left to right) for the same region. These elemental maps confirm that the bright areas in the HAADF STEM image are indeed Ti atoms, surrounded by S. In the part of the The O signal is predominantly localised at the edge (dotted region). Some C contamination can be seen, especially at the edge. b) FFT of Survey image in (a) (gamma set to 0.4 to suppress the background) and c) the EELS sum-spectrum used to produce the elemental maps presented in (a). The low signal to background ratio at the O edge explains the noise in the O-map..