In this study, we present a Monte Carlo inversion procedure dedicated to the determination of particle size and density from extinction measurements. In order to check the validity of the method, a ray tracing code able to calculate the exact extinction coefficient for a given particle size distribution was developed. The inversion method was then applied to this numerically obtained results in order to retrieve the initial particle size distribution. We found that the method provides good estimations of the particle sizes and density. Finally, the inversion procedure was applied to experimental data obtained with visible and infra-red spectroscopy. A comparison between the calculated particle sizes and the ones measured from SEM micrography shows good agreements.