The present work describes a new sensing system for the detection of photosynthetic herbicides in water based on the use of the reaction centre (RC) isolated from Rhodobacter sphaeroides, a purple bacteria. RC is a trans-membrane protein complex with one characteristic: stationary and excited states have different absorption properties. The presence of herbicides affects the path followed by the protein to return to the stationary state. Therefore the herbicides could be detected by monitoring the temporal changes of absorption following optical excitation. An optoelectronic system has been realised for this purpose. A diode at 860 nm and a hybrid photodetector are used as excitation source and detector, respectively. Optical fibres couple the optoelectronic system to a 5 cm-long cell containing the RC solution. The system has been used for the detection of five different photosynthetic herbicides: diuron, atrazine, terbutryn, terbuthylazine and simazine. The detection limit of every herbicide has been obtained: 0.5 μM for terbutryn, 1.0 μM for atrazine and terbuthylazine and 10 μM for diuron and simazine.