Method and apparatus for monitoring hydrocarbon-based fuel properties by measuring the presence and amount of fuel component molecules in the fuel solution. Detection is made of absorption related to signature vibrational modes associated with the fuel component molecules when excited in the mid-IR. In one embodiment, a mid-IR light source illuminates the fuel in-situ and a narrow bandwidth detector associated with a vibrational mode signature detects absorbance as an indication of the presence and quantity of the fuel component in solution. From the determined fuel component quantity and known characteristics, the fuel solution properties are predicted. In one embodiment, octane rating and vapor pressure for a fuel solution is determined in-situ and in real time.