The surface plasmon resonance of small metal particles is highly sensitive to the refractive index of the surrounding medium. However, for particles dispersed on a glass surface in a polymer matrix, the effective refractive index is ill-defined. The effects of the substrate and solvent refractive indices on the surface plasmon resonance of individual gold nanoparticles (prisms and decahedra) have therefore been investigated by dark field microscopy. It is found that for any given substrate, e.g., glass or quartz, the surface plasmon band red-shifts when the solvent refractive index is increased as expected. However the sensitivity is reduced from that observed in a homogeneous medium, and the effect of the supporting polymer matrix is found to be of great importance. The effective refractive index dispersion relationship follows an equation of the form n(eff) = (1 - alpha)n(sub) + alpha n(med) where the value of alpha was found to vary from 0.10 to 0.65 depending on whether the particles are embedded in a polymer matrix or deposited directly onto the substrate.