Clark type oxygen microsensors were developed for measurement of photosynthetic light response in cyanobacteria isolated from epilithic microbial communities colonizing Roman hypogea. Microelectrodes were built using polyester or teflon insulated platinum wires as working electrodes and silver wires as reference electrodes, then inserted in glass capillaries previously coated at one end with a silicon gas-permeable membrane. Platinum was polarized at −750 mVvsAg/AgCl, and 0.1 M KCl was used as supporting electrolyte. Stability, sensitivity and reproducibility of the probes were tested. These cost effective sensors exhibited comparable behaviour with those commercially available. Photosynthesis of selected cyanobacterial species was followed by monitoring the oxygen evolution at increasing irradiances (P/I curves).