An optical fibre surface plasmon resonance (SPR) sensor has beendeveloped for the detection of hydrogen leakages. A thinpalladium layer deposited on the bare core of a multimode fibrewas used as the transducer. In this device, modification ofthe SPR is due to variation in the complex permittivity ofpalladium in contact with gaseous hydrogen. This effect is enhanced by using selective injection of high-order modes inthe fibre via a collimated beam with non-normal incidence on the inputend of the fibre. Measurements of concentrationsas low as 0.8% of hydrogen in pure nitrogen have been found to bepossible. The response time varies between 3 s for pure hydrogenand 300 s for the lowest concentrations. Such a large range canbe explained by the two different crystallographic phases ofthe palladium-hydrogen system. Moreover, the response of thesensor is dependent on the length of the sensing area. Inpreliminary experiments, it has been possible to split thesensing area in order to achieve a two-point detection device.