A method for monitoring natural gas leaks was designed based on the highly sensitive measurement of oxygen, which would be displaced in the event of a subsurface volatile gas leak. A fiber optic sensor was made using a gas-permeable polymer cladding, doped with oxygen-responsive porphyrin. The evanescent field at the interface of the core and the cladding was used to detect gas leaks. Tests were carried out using methane gas leaks in a chamber filled with dry and wet sand. Results of sensitivity, response time and reversibility are presented.