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Active layers for gas sensing applications have been obtained by adding oxygen plasma functionalised multi-walled carbon nanotubes (MWCNTs) to WO₃ and using the drop-coating deposition method. Scanning electron micrographs showed that MWCNTs are well dispersed on the WO₃ matrix. X-ray photoelectron analysis showed that by adding MWCNTs to the WO₃ films the contribution of the doublet related with surface defects to the total W 4f signal increased. This gives evidence that the surface area of WO₃ films increases due to the addition of MWCNTs to the film. Two different ratios of MWCNTs in WO₃ (1/100 and 1/1000) were considered and the response of these sensors towards toxic gases such as nitrogen dioxide, carbon monoxide and ammonia was compared with that of WO₃ and MWCNTs gas sensors. The results indicate that the detection at ambient temperature of these toxic gases using WO₃ …