A fast responding fiber-optic microsensor for sensing pCO(2) in marine sediments with high spatial resolution is presented. The tip diameter varies typically between 20 and 50 mu m. In order to make the pi-I-indicator 8-hydroxypyrene-1,3,6-trisulfonate soluble in the ethyl cellulose matrix, it was lipophilized with tetraoctylammonium as the counterion [HPTS-(TOA)(4)]. The microsensor was tuned to sense very low levels of dissolved carbon dioxide which are typically present in marine systems. The detection limit is 0.04 hPa pCO(2) which corresponds to 60 ppb CO2 of dissolved carbon dioxide. A soluble Teflon derivative with an extraordinarily high gas permeability was chosen as a protective coating to eliminate interferences by ionic species like chloride or pH. Response times of less than 1 min were observed. The performance of the new microsensor is described with respect to reproducibility of the calibration curves, dynamic range, temperature behavior, long term stability and storage stability. The effect of hydrogen sulfide as an interferent, which is frequently present in anaerobic sediment layers, was studied in detail.