The first decay time-based fiber optic pH microsensors are presented which are without cross-sensitivity to molecular oxygen. They are based on radiationless energy transfer from the donor ruthenium(II)-tris(1,10-phenanthroline) to a pH-sensitive sulfonephthalein acceptor dye. The microsensors have decay times in the microsecond range, a fact that makes them compatible with existing devices designed for optical oxygen microsensing. The sensor tips have diameters of 20 to 30 μm. Outstanding features are the excellent mechanical stability and the inertness to quenching by oxygen. The dynamic range depends on the pKa of the selected pH indicator. The sensors were characterized with respect to the dynamic range, response, storage stability and photostability. A high resolution pH measurement in a marine sediment core serves as an example for its utility.