Continuous progress in the methods of preparation of new materials for sensing of toxic gases has attracted considerable interest due to environmental problems. In the present investigation, poly(methylmethacrylate)/ polyaniline (PMMA/PANI) blend nanofibres were prepared by electrospinning and dip-coating polymerisation techniques. The semiconducting behaviour of PMMA/PANI blend was found to be highly sensitive for toxic gases like NH3 and HCl at room temperature suggesting the PMMA/PANI blends as a potential material for effective gas sensing in the environmental monitoring safety systems, chemical industry, automotive industry and medical application areas. Fourier transform infrared (FTIR) spectra clearly show the characteristic peaks indicating the presence of quinoid and benzenoid rings of PANI confirming the formation of PMMA/PANI blend. Clusters of PANI on the uniform fibres of PMMA can be seen in the SEM images. Good thermal stability of PMMA/PANI nanofibres showed to be highly sensitive with fast response and recovery on exposure to NH3 and HCl gases. The phenomenon of increase in sensitivity with the increase in concentration of NH3 and HCl gases was found to be adsorption dominated indicating that the PMMA/PANI blend nanofibres are more sensitive to the surrounding due to their increased porosity and high aspect ratio. The sensing mechanism of PMMA/PANI blend nanofibres on account of more protonation and de-protonation due to exposing towards HCl and NH3 gases respectively is systematically explained in the present investigation.