The intrinsic nature of elastic light scattering by aerosol particles-the non-monotonic dependence of the scattered intensity vs. size-influences the performance of practically all single optical particle spectrometers resulting frequently in substantial erroneous sizing of particles. As a possible solution for this problem we developed a measuring system and data reduction procedure allowing simultaneous determination of particle size and its complex refractive index. The system contains two laser sources with different wavelengths and four detectors collecting scattered light in four spatially separated angular ranges. The targeted size and refractive index ranges of this device cover the size from 0.1 to 10 mu m, the real part of the refractive index from 1.1 to 2, and the imaginary part of refractive index between 0 and 1 corresponding to the properties of ambient aerosol particles. The numerical performance study shows that the sizing error of the method remains less than 10% independent of the refractive index of the particles. The assessment of the real and imaginary part of the refractive index is possible within about 15%. Experimental results verify the feasibility of this new instrumental approach showing the potential of the method to determine in real-time the size and complex refractive index of single aerosol particles without the necessity of instruments' recalibration for varying particle materials. (c) 2007 Elsevier Ltd. All rights reserved.