A new experimental method has been established for the accurate determination of the nonlinear optical susceptibility tensor ratios (chi(ZZZ)((2))/chi(ZYY)((2)) and chi(XXZ)((2))/chi(ZYY)((2))) of molecules at solid-liquid interfaces using transmission second-harmonic generation. Experiments have been carried out with Crystal Violet, Nile Blue, and D289 molecules at the CH3CN/quartz interface. The results have shown the excellent agreement with the theory of nonlinear optics. It has been demonstrated in this paper that for different molecules the phase angles of these two tensor ratios are quite different. Both the absolute values and the phase angles of the two tensor ratios can be obtained from the intensity ratios I-P-->P(2 omega)/I-S-->P,T(2 omega) and I-45 degrees-->S,T(2 omega)/I-S-->P,T(2 omega) obtained at different angles of incidence and different interfacial densities.