A phenomenon of supramolecular solvents formation in aqueous solutions containing primary amine and monoterpenoid compound is presented for the first time. A novel supramolecular solvent-based liquid phase microextraction was developed. In an aqueous phase primary amine formed isotropic solution due to formation of amphiphile supramolecular assembly. The phase separation was achieved by a coacervation process. The monoterpenoid phenol (thymol) acted as a coacervation agent and induced spontaneous in situ formation of tiny supramolecular solvent droplets. The applicability of the proposed microextraction procedure was demonstrated for the separation of sulfonamides from biological fluids (human plasma and serum). An obtained composition of supramolecular solvent (1-decylamine, thymol, water) promoted satisfactory extraction of polar analytes from aqueous phase. The calibration graphs were linear over the concentration ranges of 0.06–50 mg L−1 for sulfamethoxazole, sulfamethazine and sulfapyridine. The limit of detection calculated from the blank tests based on 3σ was 0.02 mg L−1 for all analytes. The total analysis time was 15 min.