Abstract: A porous glass optical fiber sensor, a method for its use, and a method for its manufacture. The sensor includes a glass optical fiber for conveying light, the fiber having a surface of interconnected and permeable chambers within the fiber, for optically sensing within the chambers. An indicator can be applied to the surface for sensing. The sensor is made by: first, drawing the fiber; second, heat treating the fiber to induce phase separation; and third, leaching a phase from the fiber. Thereafter, an indicator, such as a pH or moisture indicator, can be applied to the fiber for sensing. The sensor is used in conjunction with a light source, a light detector, and means for measuring change in the light caused by an agent within the porosity of the sensor.