We developed a quasi-elastic laser scattering method and applied it to the density estimation of liquid/liquid (alcohol/water, carboxylic acid/water) interfacial regions. The observed densities are smaller than the expected values in most of the cases, indicating that the interfacial densities are smaller than the bulk phase densities. Moreover, then is a marked tendency for the observed density to become smaller as the hydrophobic group in the alcohol and carboxylic acid molecules becomes larger. The results are consistent with theoretical predictions using the molecular dynamics method. The interfacial structure is discussed in connection with these results.