Abstract A novel distributed optical fibre pH sensor is reported. The principle of operation of the system is based on the detection of time-resolved fluorescence originating at spatially separated sensor sites. Sodium Fluorescein is used as the pH indicator. The fluorophor is immobilised in porous sol-gel films laid over the fibre core. The distributed system has a spatial range of 100 m and a resolution of the order of 1 m. The pH range of the system is observed to be pH 7–11.