A new type of hydrogen sensor is reported in which an optical fiber is used as the sensing element. The fiber is coated with palladium which expands on exposure to hydrogen. This changes the effective optical path length of the fiber, which is detected by interferometric techniques. Preliminary experiments have demonstrated the effect and suggest a high sensitivity and a wide dynamic range for this kind of sensor. The experimental results are compared to calculated optical path length changes. Application of this kind of sensor to the detection of other chemicals appears feasible.