The determination of the mean size distribution for a Type Ib Polar Stratospheric Cloud ("PSC") has been performed for the first time using extinction,backscattering ratios as obtained from LIDAR data at 355, 532 and 750 nm. The multiwavelength Lidar was operated in Sodankyla, Finland, during the Second European Stratospheric Arctic and Middle-latitudes Experiment (SESAME, November 1994-April 1995). It produced vertical profiles of PSC. Log-normal distributions of supercooled ternary solution (STS), composed of H2SO4-HNO3-H2O particles with mode radius of 0.68-0.9 mu m and standard deviation of 1.1-1.4, were retrieved as best fit to experimental data. Laboratory measurements of n(r) (real part of refractive index), in the 404-633 nm range, show that it depends on the water/acid ratio without any distinction between binary and ternary solutions of nitric and sulphuric acid.