Optodes based on bromothymol blue (BB) and bromocresol purple (BP) acid-base indicators were constructed. The dyes were chemically immobilised on the surface of a bifurcated glass fibre optical bundle, which was previously silanised with 3-aminopropyl-triethoxysilane. A PTFE/acrylic cell was specially constructed, allowing the use of the optodes as detector in flow analysis systems. The optodes were evaluated for their pH responses in different flow approaches, such as continuous flow, flow injection and monosegmented flow systems. A steady state signal was achieved in the continuous flow system. The effect of the flow rate and the injected volume on the signal amplitude were studied. Signals up to 69 and 77% for BB optode and up to 82 and 92% for the BP optode of the steady state signal can be obtained by employing flow injection and monosegmented flow systems, respectively. The sampling frequency can be improved and sample consume can be lowered by employing the monosegment flow approach. © 1998 Elsevier Science S.A. All rights reserved.