Picosecond excitation-probe measurements using a far-infrared free-electron laser (CLIO) have revealed large nonlinearities for indium antimonide at 4.7 µm. A theoretical model is described to determine the cw third-order nonlinear susceptibility and the interband relaxation time of the semiconductor which were found to be -8.6×10-11 m2 V-2 and 0.3 ns, respectively. Furthermore, the observation of the associated coherent transient grating effects allows us to obtain a coherence time of the laser system (2.5 ps) and the ?(3) of the transient grating which was found to be -7.2×10-13 m2 V-2. The measurements were performed at room temperature on undoped bulk InSb. © 1995 American Institute of Physics.