A laser light scattering technique was used for the determination of mutual solubilities at moderate pressure (saturated vapor pressure of a mixture) in four high-viscosity binary systems: aniline + methylcyclohexane, phenol + heptane, phenol + octane, and glycerol + 1-pentanol. The liquid-liquid solubility data were measured up to the upper critical solution temperature. The experimentally determined solubility data were satisfactory correlated by the NRTL equation and Tsuboka-Katayama's modification of the Wilson equation.