Abstract An evaluation of the feasibility of a fiber-optic pH sensor has been carried out. The pH sensor is composed of an optode connected to the transmitter-receiver apparatus (Optolec H) through a 100 m long fiber. Three diodes of carefully chosen wavelengths are located in the Optolec H and emit through a bundle of nine fibers to the end of the optode, where the oncoming light is reflected on an immobilized pH-indicator-based spherical bead of resin and sent back for analysis to the Optolec H through a central fiber. The influences of ionic strength as well as precision, response time, and lifetime have been determined. This device is to be used for routine in situ measurements in an underground laboratory devoted to studies in connection with nuclear waste repositories, where harsh conditions are encountered for direct collection of waters and traditional post-determination with a pH meter.