The patented fiber optic chemical sensor (FOCS)™ technology is based on the principle of modulation of light transmitted along an optical fiber. A portion of the cladding of a small segment of the optical fiber is replaced by a proprietary chemical coating, which changes the refractive index in the presence of hydrocarbons. This change in the index is proportional to the concentration of hydrocarbon present. The change in the index alters the light propagation properties of the fiber, thereby altering the amount of light transmitted. A small change in refractive index can yield large changes in the light transmitted through the fiber. The change in the transmitted light can be calibrated, leading to a sensor that detects low levels of hydrocarbons. The probe, located at the end of a cable, can be installed in a monitoring well or other locations where dissolved hydrocarbons are expected to be present.