We report on a method to determine the temperature coefficient of the refractive index (dn/dT) for non-fluorescent liquids which is based on the experimental observation of conical diffraction induced by self-phase modulation (SPM). Light absorbing impurities were added to allow for heat deposition by the excitation laser beam. We studied samples of tetrahydrofuran (THF, C 4 H 8 O) and toluene (C 6 H 5 CH 3) with suspended cadmium selenide/zinc sulfide (CdSe/ZnS) core–shell quantum dots as absorbing impurities and samples of water (H 2 O) and THF with a small addition of blue-black commercial ink. The well-known thermal lens (TL) technique was also employed in this work for the sake of comparison with the proposed conical diffraction pattern. Thermo-optical parameters obtained by the mode-mismatched thermal lens (TL) technique validated the SPM measurements of dn/dT. In addition, we applied in this work the Fresnel–Kirchhoff diffraction formalism for the theoretical prediction of the rings that are observed in the far-field region. The technique proposed herein to determine dn/dT may serve as a fast, cost-effective optical method for evaluating adulterated or impure chemicals.