Determination of phenols in crude oils and derived fuels requires a sample pretreatment step, usually performed by liquid-liquid extraction or preparative chromatography. In this work, sample preparation is accomplished using a silicone membrane separation unit coupled on-line to a high performance liquid chromatograph with amperometric and ultraviolet detection. The contents of phenol, cresols and dimethylphenols were determined in thirty three samples including three crude oils, twenty gasolines and ten diesel fuels. The whole set-up is fully automated through a feed-back system that allows the microcomputer controlling the process to examine the signals in real time and to make decisions while the experiment is running.