We applied two nonlinear optical techniques for investigating CH3OH-H2O and CH3CN-H2O binary liquid mixtures. The measured third-harmonic susceptibility of CH3OH-H2O with varying methanol mole fraction sensitively reflects the microstructure of the liquids. The perturbed polar distribution of methanol molecules by interfacial water at the vapor-liquid interface of CH3OH-H2O was revealed with infrared-visible sum-frequency generation (IVSFG). For CH3CN-H2O our third-harmonic-generation result clearly indicates the existence of microheterogeneity in a mixture with a mole fraction of acetonitrile larger than 0.3. By comparing with an IVSFG study of CH3CN-H2O reported by Eisenthal et al. [J. Chem. Phys. 98, 5099 (1993)], the observed critical acetonitrile concentration for the onset of phase separation in CH3CN-H2O appears to be larger than that at the liquid surface.