Traditionally microscale measurements of pH are based on potentiometric measurements with a pH glass microelectrode. The preparation of these electrodes is, however, very time consuming. We developed pH micro-optodes for use in seawater in the range of pH 7 - 9. The optodes are based on immobilized acid-base indicators, which change their color and/or fluorescence properties as a function of the pH. Various dyes were immobilized directly on the tip of a tapered optical fiber by different techniques. We then investigated these pH optodes with respect to response time, mechanical stability and calibration characteristics. Dependent on the optical properties of the indicator material we used different light emitting diodes (LED's) as the light sources and either a photodiode or a photomultiplier as detector.