Abstract: Sensing the presence of furfuraldehyde (FFA) in oil is effected by a porous solid sensor (10) comprising aniline acetate entrapped in an inert matrix. The matrix may be a sol-gel preferably based on methyltrimethoxysilane. The solid sensor (10) reacts with FFA to form a pink colored complex which has a characteristic which correlates with the concentration of FFA in the oil and which can be measured photo-optically. Photo-optical measurement may be by absorption or fluorescence when the sensor is irradiated with an interrogating beam. The absorption maximum wavelength is about 525 nm and irradiation at that wavelength produces a maximum fluorescent output at about 570 nm.