A versatile interface light-scattering (ILS) spectrometer for the detection of light scattered by thermally excited capillary waves at fluid interfaces has been developed. The instrument permits observation over a wide range of capillary mode wave numbers, using scattering cells with cylindrical geometry. Owing to the cylindrical geometry, it is possible with this spectrometer to perform combined interface and bulk light scattering experiments using the same cell. The performance of the experimental set-up, including a sapphire-tube high-pressure piston cell, has been assessed through measurement of the interfacial tension in a system of coexistent, mutually saturated phases of water and 2-butanol. The results obtained at ambient pressure were shown to be in excellent agreement with those previously obtained with the use of an experimental set-up including a planar optical geometry scattering cell. The light scattering measurements demonstrate one particularly attractive feature of this method: Not only is it a nonperturbative and absolute method for determining interfacial tension, but it also provides a way to check the reliability of the results. Consistency of results over a wide range of capillary mode wave numbers provides assurance that no biases exist either in the experimental system or in the data analysis. A preliminary study of a six-component hydrocarbon model gas-condensate system, which in certain respects resembles real gas-condensate systems, demonstrates that reliable information can be obtained on the interfacial tension and the sum of the gas and liquid shear viscosities. The prospects of using the ILS method for more systematic studies of such systems appear very good. (C) 1994 Academic Press, Inc.