A novel fiber optic pH sensor is reported. The sensor is based upon surfacic adsorption of methylene blue dye, producing absorption in the evanescent field surrounding the sensing fiber. The sensor is based upon a 200 μm diameter Plastic Clad Silica (PCS) fiber which has its cladding removed in order to expose its core. The linear range of operation is between pH 3 and pH 9 and its response time is considerably shorter than indicator based probes which measure spectral changes of pH sensitive chemicals. Effects of measurand ionic strength are shown to be negligible which is a further important advantage of this sensor over other optical fiber pH sensors. The sensor design is sufficiently flexible to allow it to be configured in probe (reflective) or in-line (transmissive) form. The latter allows the application of time domain reflectometry technique which may be used to achieve multipoint (distributed) sensing.