Abstract: A fuel contamination detection system for use in testing the contamination level of a fuel sample, wherein the system is selectively operable in a first mode as a particulate detector and in a second mode as a free water detector. A filter support slide includes two apertures and is adapted to support different sizes of filter elements over the apertures. In the particulate detector mode, the slide carries a pair of filter elements through which a fuel sample has been passed, wherein one filter element bears particulates from the sample. A particulate detection circuit includes an incandescent light and a photocell, between which the slide is moved to introduce one filter element and then the other, whereby a differential reading of their respective opacities is obtained and displayed. In the free water detector mode, only one aperture of the slide is used to carry a chemically treated water filter pad.