Benthic aquatic environments like biofilms or sediments are often investigation by measuring profiles of chemical or physical parameters at a high spatial resolution (< 50 micrometers ). This is necessary to understand e.g. transport processes and the biogeochemistry of the sediment water interface. A variety of electrochemical and optical microsensors has been developed and used for this purpose. In most of these applications the temperature of the investigated biofilms or sediments is assumed to be constant. However measurements with thermocouples of an appr. diameter of 300 micrometers have shown that this is not always the case for illuminated shallow water sediments and biofilms. We developed new microoptodes for measuring temperature distributions at a high spatial (< 50 micrometers ) and thermal (< 0.2 degree(s)C) resolution in aquatic systems. The new sensors are based on a fluorophore that is well known for its application in oxygen sensing-Ruthenium(II)- tris-1,10-phenantroline. Demas et al. (1992) discussed the possible use of highly luminescent transition metal complexes as temperature indicators. We have approached this idea from our experiences with ruthenium complexes as oxygen indicators. The first realized sensor consists of a closed microcapillary filled with an indicator solution and in inserted tapered optical fiber. The principle uses the temperature dependence of the fluorescence lifetime in the solution. To keep the solution oxygen free an oxygen scavenger is added to it. The change of the lifetime is detected by a special measuring device that uses a phase modulation technique.