This paper describes a method to determine the optical properties of submicron-coated aerosol particles with metallic core. The particles were prepared using aqueous suspensions of colloidal metal particles. After spraying of the suspensions and drying the spray, the residual aerosol particles consisted of a metallic core of well-defined size and a dielectric salt shell of unknown thickness, which formed during the drying process. It was shown that the thickness of the dielectric shell could be determined within an accuracy of 1 nm from evaluation of optical extinction measurements on the aerosol particles in the wavelength range between 300 and 1000 nm. Furthermore, the contribution of salt particles to the aerosol extinction spectra could be resolved from the contribution of the coated particles. For measurement in this spectral region, a White cell with 32 m path length was constructed and combined with a multi-channel spectrometer.