This paper discusses the investigation of the complex third-order nonlinear optical susceptibilities to obtain spectroscopic information on one- and two-photon resonances. The merits of the new method of phase-tuned optically heterodyned femtosecond Kerr gate relative to the inner reference method are discussed in relation to obtaining the magnitudes and the signs of both the real and the imaginary components of the third-order optical susceptibility. The results presented here show that, as expected, near a one-photon resonance, the saturation effect leads to a negative imaginary component while near a two-photon resonance the sign of the imaginary component is positive.