In this study, we present a new chemical sensor based on a functionalized porous silicon and sensitive to dimethyl methyl phosphonate (DMMP), a simulant of sarin gas, known as one of the most toxic warfare agents. The porous silicon was prepared by anodising a boron-doped p-Si (100) wafer. A metal oxide TiO 2 layer was then deposited by thermal evaporation of Ti and oxidised under atmospheric conditions to form a TiO 2 film which was used as a reactive coating. The characterization of the PSi and TiO 2 /PSi films was investigated using SEM, EDX and XPS spectroscopies. The results indicated that the TiO 2 was well incorporated on the surface and slightly inside the porous silicon layer. FT-IR studies were then carried out before and after exposure to DMMP and the detection performance was compared with PSi. The results obtained showed that our surfaces have the ability to detect DMMP down to 0.5 ppm compared to uncoated PSi layer. We have also shown that a TFA pre-treatment of our TiO 2 modified PSi, prior the exposure to DMMP, is crucial for increasing its sensitivity and that our sensor can be reused several times with a simple hexane cleaning step, a key criterion for routine applications.