The concept of distributed pH measurement was demonstrated at a sensitized region 4 m from the distal end of a 20 m length of plastic optical fibre. The cladding was removed from the fibre over 150 mm and the bare core was exposed to an aqueous solution of methyl red at three values of pH, between 2.89 and 9.70. The optical fibre was interrogated at 648 nm using a Luciol photon counting optical time domain reflectometer, showing the sensing region was attenuated as a function of pH. Least-squares analysis of the traces yielded the linear equation Y ≤ -1.35 pH + 11.4 with a correlation coefficient of -0.96 indicating a high degree of correlation, permitting pH measurement to an estimated 1.0 pH at the sensing region. Chemical sensing measurements by photon counting OTDR using a standard PCS fibre in aqueous (0.0010 M) and ethanolic (0.000 65 M) solutions of methyl red at 657 nm are also presented. The ingress of methyl red in ethanol was detectable after 2 h and showed reversibility to the pre-doped condition in under 7 min. A case for a silicone resin clad PMMA core hybrid optical fibre as a chemical or pH sensor is presented. \circledC 2007 IOP Publishing Ltd.