The construction of a pH sensor, having the configuration of a probe, is described. The probe utilises a plastic-fibre bundle at the end of which is fabricated a sensitive tip consisting of a styrene-divinylbenzene copolymer supporting an adsorbed colorimetric pH indicator, which is retained in position by a membrane of polytetrafluoroethylene (PTFE). Changes in pH in the vicinity of the sensitive tip cause a variation in the attenuation of specific reflected visible radiation bands. Initial results and performance specifications using bromothymol blue as the colorimetric indicator are presented.