Diphenyl ether is a potential modifier to decrease the melting temperatures of wash;oils and thus to prevent solid formation from the recycled stream in an aromatics removal process. Fundamental study of the solid-liquid equilibria for the related mixtures was undertaken in the present work. Solid-liquid equilibrium data were measured for binary and multicomponent mixtures containing model compounds of wash oils (acenaphthene, dibenzofuran, fluorene, and phenanthrene) and diphenyl ether. The results showed that adding diphenyl ether substantially lowered the melting points of wash oils. A series of batch distillations with various initial feeds were also made to investigate further the property changes of the wash oils before and after recovery. It was found that diphenyl ether could still remain in the residues (the recovered wash oils) and changes in the melting temperature of oils were minor. These low-melting wash oils can be recycled directly to benzol scrubber for reuse. The binary solid-liquid equilibrium data were correlated with the NRTL model. The solution model with the determined parameters is capable of predicting the solid-liquid equilibrium temperatures of the pseudo-binary (multicomponent) system to an AAD of 0.3%.