We present an experimental study on measurement of nonlinear refractive index ($n_2$) of organic liquids when the thermo-optic effects manifest into large nonlinear phase shifts ($\Delta\phi_0$) in an open-aperture 𝑍-scan configuration. Although we do not obtain the familiar peak–valley normalized transmittance curve as in the case of closed-aperture 𝑍-scan technique, we use a theoretical model using Gaussian beam decomposition (GD) technique to estimate the value of $n_2$. Using this recipe, we obtain the nonlinear refractive index $n_2 = −(4.90 \pm 1.20) \times 10^{−15}$ cm²/W for toluene (organic solvent) and $n_2 = −(10.60 \pm 2.10) \times 10^{−15}$ cm²/W for an organic polymer solution (10$^{−4}$ Min toluene). By carrying out absorption measurements directly with an unfocussed Gaussian beam, we found nonlinear absorptions $\beta_{\text{tol}} = (2.42 \pm 0.20) \times 10^{−13}$ m/W and $\beta_{\text{poly}} = (2.79 \pm 0.24) \times 10^{−13}$ m/W which are close to the expected results.