The real and imaginary parts of individual tenser elements of the nonresonant third-order nonlinear optical susceptibility of simple liquids, carbon tetrachloride and benzene, have been characterized for the first time by transient grating optical heterodyne detected impulsive stimulated Raman scattering (TG-OHD-ISRS). Optical heterodyning is achieved through additional scattering induced by a thermal grating. Vibrational modes to 1000 cm(-1) are impulsively prepared and probed using 15 fs laser pulses. This technique allows for a precise determination of frequencies and dephasing times of intramolecular Raman-active vibrations. It is shown that the symmetry of these modes defines the measured phase of the oscillatory modulations of the various linearly detected tenser components of chi((3))(t). Advantages to conventional techniques, such as optical heterodyne detected Raman induced Kerr effect spectroscopy or homodyne detected impulsive stimulated Raman scattering, are demonstrated. Further detection of the real or imaginary part of the susceptibilities using phase locked femtosecond optical pulses is discussed.