The most known problem of separate sewer systems is the existence of illicit connections and, hence, the deposition of untreated sewage to the receiving waters or the occurrence of urban flooding due to overloading of wastewater systems with storm water. Although there are several known and relative expensive techniques for inspection of storm sewer systems (e.g. smoke or sound based technologies), most of them concentrate on the structural or operational reliability of the inspected pipe and ignore the conditions that prevail beneath the surface of the water, detecting only some defects. An infrared camera (Flir A35) was used for a more comprehensive approach towards the inspection of a storm sewer pipe, by identifying abnormal thermal finger prints (due to lateral connections) along a flume. The illicit connections were simulated by discharging warm or cold water through several types of lateral connections (variation of diameter, intrusion, depth, etc.). Data analysis revealed that the detection and quantification of illicit connections is possible, albeit under certain conditions.