A laser aerosol size spectrometer (LASS), an "Owl" and a spiral centrifuge were utilized to measure the distribution of diameters of airborne droplets of di-2-ethylhexyl sebacate (DES). The LASS and the centrifuge were calibrated with polystyrene spheres. When DES aerosols were measured the refractive index and the density of DES were taken into account. The modal diameters evaluated with the optical instruments agree very well; the centrifuge gives slightly lower values. The influence of density, refractive index and aerodynamic drag on the size evaluation is discussed. Aggregates of uniform polystyrene spheres served as irregular shaped particles. In contrast to the centrifuge the LASS measures the equivalent spherical volume diameter independent of the particle shape.