CO adsorption on structurally very different electrocatalysts, namely carbon supported Pt catalysts and adsorbed colloids on surfaces (a model system), has been studied. From a comparison of the spectra of CO monolayers on these disparate electrocatalysts, conclusions can be derived regarding the structure of the particles. A correlation between CO vibrational frequency and structural properties, e.g. isolated particles versus agglomerated particles, is found. A dependence of vibrational frequency on the particle coverage is assigned to an increasing agglomeration with increasing Pt-coverage. The spectra of the technical electrocatalysts can be explained in terms of the simpler model systems.