Abstract: A process for spectroscopically measuring the content of a component in a liquid mixture involves passing a single optical beam through the mixture without the use of a reference beam and measuring the intensities of transmitted light in two different wavelength bands by means of two adjacent detectors responsive to light in the two wavelength bands respectively. The component being measured absorbs light within one of the wavelength bands. The ratio of the detector outputs can then be related to the component concentration. The principal use of the invention is for in-situ measurement of methanol in gasoline for motor vehicles.