The removal of process of oleic acid from fibrous substrates in aqueous solutions of sodium dodecyl sulfate (SDS) has been investigated using differential scanning calorimetry (DSC). The DSC heating curves for the fibrous specimens soiled with oleic acid and then washed with an aqueous solution of SDS have shown two distinct endothermal peaks; one is due to the melting of ice adhering to the specimens, and the other is due to the melting of oleic acid remaining on the specimens. A broad endothermal peak attributable to a liquid crystalline phase composed of SDS, water, and oleic acid on the specimens in the washing process has also been observed between the melting peaks of ice and oleic acid. The change of DSC curves with washing time suggests that the formation of a liquid crystalline phase would make some contribution to the removal of oleic acid.