A Monte Carlo method is applied to simulate the extinction of a laser beam collimated on a suspension of Ay-ash particles. The objectives are to evaluate the method by comparing predicted angular distributions of the scattered intensities with measurements available in the literature, to investigate the influence of multiple scattering on these measurements and to estimate the sensitivity of the results on the complex refractive index of the medium. The Lorenz-Mie theory is used to model the radiative properties of the particles. The examined cases showed that their scattering behaviour is sensitive on the imaginary part of their refractive index, for which widely varying values are available in the literature. The proportion of multiple scattering in the total scattered intensity, as received in various angular positions, is calculated. That allows the estimation of angular regions where this proportion is minimised. (C) 2001 Elsevier Science Ltd. All rights reserved.