We describe a surface laser‐light scattering (SLLS) spectrometer, which uses optical heterodyne detection to measure the temporal behavior of the thermally generated capillary waves at a fluid/fluid interface. From the measurement the interfacial tension and one of the bulk fluid viscosities are calculated. An instrument resolution model is described that characterizes the resolving power of the SLLS spectrometer based on the optical parameters measured at the photodetector. The instrument resolution is defined and measured, and the resolution is adjusted to optimize the signal‐to‐noise ratio. The model was verified using light‐scattering measurements with wave numbers in the range 250–1750 cm−1and with instrument resolution varied by more than a factor of 2. Measurements were made on several liquids whose viscosities vary over an order of magnitude. Data reduction used the analysis of thermally generated capillary waves recently published [A. Hajiloo and J. C. Slattery, J. Colloid Interface Sci.112, 325 (1986)]. The values calculated for both surface tensionandliquid bulk viscosity were in good agreement with the literature.