Self-assembled monolayers (SAMs) of 2-aminoethanethiol (AET) or 1,6-hexanedithiol (HDT) are formed on An and Ag substrates, and colloidal An (13.6 +/- 2.0 nm diameter) and Ag (30.0 +/- 10 nm diameter) nanoparticles are deposited onto these SAM surfaces. The resulting multilayered nanostructures, prepared with different combinations of the substrate, SAM, and nanoparticles, are studied with surface plasmon resonance (SPR) measurements. Under certain circumstances, the SPR sensitivity of the bare metal is considerably enhanced by the presence of the metal particles. Both the occurrence and the degree of this effect depend strongly on the choice of the combination of materials in the multilayered system. The substrate/SAM/nanoparticles systems that exhibit the enhanced SPR sensitivity include Au/AET/Au, Au/HDT/Au, Au/AET/Ag, and Ag/AET/Ag. On the other hand, the effects of the nanoparticles are relatively week for Au/HDT/Ag, Ag/HDT/Ag, Ag/ AET/Au, and Ag/HDT/Au. The observed system-selective nature of the nanoparticle induced SPR enhancement is discussed using Fresnel equations in a six-phase model. The results demonstrate how the interactions within and among the different components of a nanoparticle based SPR sensor affect the performance of such a sensor. These results also demonstrate the potential utility of the SPR technique in the investigation of structural and optical properties of self-assembled nanostructured materials.