The possibility of solvent molecules being trapped within the monolayer interior during the self-assembly of octadecanethiol molecules on gold was investigated by means of surface-enhanced Raman scattering (SERS). The monolayers were self-assembled from acetonitrile solution. The rationale behind the choice of this particular solvent was its characteristic strong band in the SERS spectrum around 2250 cm(-1), where there are no Raman bands in the spectra of alkanethiols. With the example of acetonitrile, we point out the possibility of trapping molecules of the solvent from which the monolayer self-assembly is carried out. Traces of solvent molecules within the monolayer can affect the electrochemical behaviour of monolayer-modified electrodes.