Abstract: The invention relates to a process and apparatus for measuring, optically and continuously, the hydrocarbon content in a liquid. An apparatus according to the invention is placed on board an oil tanker comprising a partition which separates a front compartment of inherent safety from a rear compartment. A continuous sample of deballast water from the tanks is taken via a conduit. This sample is clarified in a centrifuge, driven by a motor, which eliminates the particles in suspension. The liquid leaving the centrifuge is emulsified then it passes into an optical measuring cell comprising two optical fibers which connect the cell to an electronic cabinet placed in the rear compartment. One application of the invention is the monitoring of the hydrocarbon content of the deballast water in oil tankers.