A technique for determining the absolute size and refractive index, as well as small size and refractive index changes, of a microdroplet has been presented. The technique utilizes the wavelengths of the intensity peaks (resonances) of light scattered by a droplet observed in the planes parallel and perpendicular to the plane of incident light polarization and unambiguously distinguishes droplet growth by the formation of a layer from homogeneous growth on the basis that the formation of a layer results in substantially different spectral shifts of the scattered transverse electric (TE) and transverse magnetic (TM) mode resonances. The technique has been applied to study absorption and layer formation on single droplets exposed to a vapor of a nearly immiscible component. Wavelengths of TE and TM resonances measured by scanning with a laser are used to determine the size and wavelength-dependent refractive index of a droplet prior to exposure to the vapor. The spectral shifts of two resonances, one TE and another TM, after the exposure are used to calculate size changes due to the absorption and layer formation. The experimental data show good agreement with the theory, and the results suggest that the presence of a monolayer on a droplet can be detected by the technique.