Infrared absorption measurements at normal incidence of radiation have been carried out on octadecanethiol (ODT) deposited from ethanol-water mixed solutions onto colloidal silver particles (ca. 30 nm diameter) on Si(111) substrates. The infrared absorption spectra taken as functions of silver mass thickness and ODT concentration show that the intensities of the CH₂ asymmetric and symmetric stretch modes of ODT are twice enhanced in comparison to those observed in the absence of the particles. The spectra also reveal that at any ODT concentration the absorption intensities exhibit a maximum for a particular mass thickness of the particles, indicating that the enhancement is linked to the aggregation of the particles. Contribution of the surface plasmon resonance apparently does not play a major role in the present systems. Approximately the same magnitude of enhancement is obtained for silver hydrosol particles exposed to ODT from the solution followed by depositing on the substrates. Scanning electron morphological inspection has been done on the aggregates in relation to the enhancement.