MATRIX isolation has been widely used to trap reactive species in the cavities of an inert host for subsequent spectroscopic observation1. Occasionally, spectral features appeared which could not be ascribed to the single trapped species and had to be attributed to an aggregate. These bands were generally regarded as parasitic and their origin was not pursued. We have discovered that metal dimers can be readily formed in large quantities in matrix conditions and that the bands associated with these entities can be identified readily using a novel technique which makes use of the fact that the metal atoms being deposited are capable of diffusion either on the matrix surface or within a narrow region (the reaction zone) near the surface before its kinetic energy is dissipated sufficiently to immobilise it.