Abstract: Apparatus and methods for determining the composition of a fuel mixture comprising hydrocarbons and oxygenates, wherein more than one measurement is made of the interaction of light passing through a light guide and the fuel mixture in which the light guide is immersed using light of a variety of different wavelengths and/or light guides having different indices of refraction. Data from the measurements is then interpreted to establish the concentration of hydrocarbons in the fuel mixture and/or the concentration and identity of the oxygenates. Pursuant to one type of embodiment of the invention, a plurality of light guides is provided, with particular light guides being used for each oxygenate to be analyzed (including water). In accordance with an alternative type of embodiment of the invention, a single light guide is employed with a single wide band light source and a plurality of light sensors.