Hydrocarbon layer thickness monitors are used in oil/water separators, groundwater wells, sumps (wet), tanks and ponds to monitor the thickness of hydrocarbon and various other organic solvents. It consists of a controller (signal processor) and one or more sensors, depending on application needs. It monitors the thickness of hydrocarbons and other organic solvents on water in many applications including remediation control, skim tanks, monitoring oil/water separators and wastewater sewer monitoring. This system monitors oil thickness changes over time in groundwater wells to enable effective oil removal methods.