Structural information on rhodamine dyes adsorbed at heptane/water and hydrophobized silica/water interfaces are obtained by in situ second harmonic generation (SHG) spectroscopy. The SHG spectra provide spectroscopic evidence that the molecular association occurs at both interfaces in a different manner depending on their interfacial coverage. At the heptane/water interface, an "in-plane associate" in which the electronic transition dipoles of each monomer are arranged parallel to one another appears as the sole species present. The number of monomer units making up this associate increases with the interfacial coverage while the tilt angle of the monomer unit is kept constant. By contrast, both a "sandwich dinner" and the in-plane associate are observed at the hydrophobized silica/water interface with increasing interfacial coverage. These results successfully demonstrate differences in adsorption behavior and molecular association between liquid/liquid and solid/liquid interfaces.