A new technique for the calculation of the optical properties of agglomerated spherical primary particles is presented. By means of the so-called virtual refractive index, the multiple scattering within the agglomerate is described by a modified Mie solution, thus avoiding restrictions with respect to the primary particle size. An additional benefit of the method is the small numerical expense required, permitting practical use for complex agglomerate structures such as real combustion aerosols. Utilizing this method, attention is focused on the calculation of the extinction cross-section of real combustion aerosols in order to discuss the results of multiple-wavelength extinction measurements within combustion systems. It is found that approximately the primary particle diameter is determined. Furthermore, the error that occurs can be avoided by means of the method presented without a detailed knowledge of the agglomerate size. Thus, the present work eliminates some current uncertainties inherent in optical combustion aerosol characterization.