A method for determining size and composition of a particulate matter in a flow produced by a combustion process, the method comprising: - projecting a polarized laser light beam through the flow in a projection direction (Pd) transversal to a conveying direction (Fd); - gathering the scattered light from the flow in a sideward scattering direction and in at least one forward scattering direction (S2; S2, S3) with respect to the projection direction (Pd); - separating, for each of said scattering directions (S1, S2; S1, S2, S3), the gathered light into two polarized light components on the planes parallel and orthogonal to the scattering direction (S1, S2; S1, S2, S3); - measuring, for each of said scattering directions (S1, S2; S1, S2, 53), the light intensity of each of the corresponding polarized light components; - calculating, for each of said scattering directions (S1, S2; S1, S2, S3), a respective scattered light polarization ratio (PR1, PR2; PR1, PR2, PR3) as a function of the light intensities of the polarized light components measured for the scattering direction (S1, S2; S1, S2, S3); - determining the size of the particulate matter as a function of the scattered light polarization ratio calculated for said sideward scattering direction; and the method being characterized in that the flow is a fume flow and comprises: - determining two coefficients (A, B) by means of a linear regression of experimental test results wherein the scattered light polarization ratio related to said at least one forward scattering direction (S2; S2, S3) is determined for various particulate samples having different unburnt carbon percentages; and - determining the unburnt carbon percentage as a function of the scattered light polarization ratio (PR2; PR2, PR3) calculated for said at least one forward direction of scattering (S2; S2, S3) by means of a linear relation of the type PR = A ⋅ C + B where A and B are said coefficients, C is the unburnt carbon percentage and PR is the polarization ratio. - projecting a polarized laser light beam through the flow in a projection direction (Pd) transversal to a conveying direction (Fd); - gathering the scattered light from the flow in a sideward scattering direction and in at least one forward scattering direction (S2; S2, S3) with respect to the projection direction (Pd); - separating, for each of said scattering directions (S1, S2; S1, S2, S3), the gathered light into two polarized light components on the planes parallel and orthogonal to the scattering direction (S1, S2; S1, S2, S3); - measuring, for each of said scattering directions (S1, S2; S1, S2, 53), the light intensity of each of the corresponding polarized light components; - calculating, for each of said scattering directions (S1, S2; S1, S2, S3), a respective scattered light polarization ratio (PR1, PR2; PR1, PR2, PR3) as a function of the light intensities of the polarized light components measured for the scattering direction (S1, S2; S1, S2, S3); - determining the size of the particulate matter as a function of the scattered light polarization ratio calculated for said sideward scattering direction; and - determining two coefficients (A, B) by means of a linear regression of experimental test results wherein the scattered light polarization ratio related to said at least one forward scattering direction (S2; S2, S3) is determined for various particulate samples having different unburnt carbon percentages; and - determining the unburnt carbon percentage as a function of the scattered light polarization ratio (PR2; PR2, PR3) calculated for said at least one forward direction of scattering (S2; S2, S3) by means of a linear relation of the type PR = A ⋅ C + B where A and B are said coefficients, C is the unburnt carbon percentage and PR is the polarization ratio.