Sensor structure (16) is provided for online monitoring of levels of Furans in oil of a transformer tank. The sensor structure includes a UV light source (26), a filter (30) permitting only UV light of a certain wavelength range to pass, a window (32) permitting the filtered UV light to passes there-through and a UV light detector (36) to receive UV light that passes through the window. When the sensor structure is mounted to the transformer tank that is online so that the window is exposed to oil, and Furans in the oil are being monitored, the Furans will absorb UV light, creating a difference in UV light received by the light detector when compared to the UV light received by the light detector when the monitored oil has no Furans therein. The output signal of the light detector is substantially proportional to a total of Furans in the monitored oil.