Space and time behaviour of the refractive index in the vicinity of a soot particle being burnt by a high-power laser field was studied experimentally using shift interferometry. At the start of the combustion thermal diffusion is shown to dominate over other processes involved and the optical inhomogeneity is close to the spherical shape. Free convection is found to develop 5-10 ms after the laser irradiation. Capabilities of interferometric methods for this kind of investigations are discussed.