Using the close-aperture Z-scan technique, the pure nonlinear refractive index (n2) of carbon disulfide is measured with a 76 MHz repetition rate femtosecond laser. Strong interference of thermal effects exists with high-repetition-rate lasers that result in negative values of n2. We remove the thermal effect completely by continuously increasing the sample flow rate (F) in a sample cell as indicated by the change in sign of n2 from negative to positive. The positive value of n2 is due to Kerr-type nonlinearity. At sufficiently high values of F of >25 ml/min, all thermal effects are removed, resulting in an n2 value that matches low-repetition-rate experiments.