The optical spectroscopic technique using pH sensitive dyes has been extended to measure pH in high-temperature, pressure, and ionic strength environments. Lack of standard calibrating buffers for these conditions necessitated a two-step process for development of this technique. The first step involved chemical equilibrium modeling to predict pH values of simple buffer salt systems at high temperatures, pressures and ionic strengths. The second step involved using these modeled buffer solutions as calibration solutions for the dye chemistry and equilibrium dissociation constant. Optical spectra of dye in these buffer solutions were used to calibrate the dye equilibrium dissociation constant. Results are presented for 293 degrees K to 373 degrees K temperature range, 0.101325-65.5 MPa pressure range and 0-3 mol/kg water ionic strength range. Once the dye equilibrium dissociation constant has been characterized, pH of unknown solutions can be obtained from their dye spectra. This method has been successfully employed for high-temperature and high-pressure measurements of oilfield waters. (c) 2006 American Institute of Chemical Engineers.