Plasma deposited thin films are very promising for the development of innovative gas sensors based on plastic optical fibres (POF). POFs exploit the evanescent field interaction, so that the sensors can be produced by depositing a sensing thin film onto the fibre surface and measuring the light transmittance change because of its reaction with the gas. A standard, low-cost, commercial step-index, highly multimode POF was used to set up cumulative sensor prototypes to detect H2S vapours in low concentration. The approach proposed for the realization of the sensors foresees the surface modification of the plastic optic fibre in two steps: (1) etching of the cladding by means of ethyl-acetate and (2) deposition of an Ag nanostructured sensing film by plasma sputtering. The study of the gas diffusion/reaction process occurring onto the fibre surface is a fundamental step to arrange simple and low-cost devices characterized by high sensitivity and high selectivity towards the gas of interest. This process depends on many parameters connected to the sensing layer chemistry and morphology and are, consequently, complex and difficult to predict. The POF-based approach was employed also for monitoring the gas diffusion/reaction process through the sensing layer to model the process itself and to understand how it can affect the expected sensor response. Copyright © 2011 John Wiley & Sons, Ltd.