The effect of internal radiation on the shape of the solid/liquid interface in low and high thermal gradient Czochralski oxide processes is studied with respect to the kind of reflection of heat radiation (diffuse or specular) at the crystal side surface. Special attention is given to the interface inversion at the stage of shouldering. It is shown that in the first process variations of interface are related directly to the specular reflection, while in the second one internal radiation plays more likely an auxiliary role and variations of the shape of the crystallization front are controlled by convection. Nevertheless, in optically thin crystals with specular surface internal radiation can prevent or, at least, hamper the interface inversion. It is also shown how the back-melting process is evolved during inversion. (c) 2007 Elsevier B.V. All rights reserved.