Cavity ring down spectroscopy is widely adapted for studying light extinction by aerosol in laboratory and field studies. The complex refractive index of an aerosol can be retrieved by finding the theoretical Mie theory curve that best fits the measured extinction efficiency for as many aerosol diameters as possible. In this study we introduce a new retrieval approach for the complex refractive index of aerosols using extinction measurements at only two carefully selected size parameters. We show that for three model substances measured (ammonium sulfate, Suwannee river fulvic acid and nigrosin) and for 22 other reanalyzed datasets, the ‘2-points’ measurement approach enables the retrieval of complex refractive indices with comparable uncertainty to the traditional measurement and retrieval procedure. The advantages and disadvantages of the new approach are discussed.