Abstract During crystallization processes a redistribution of admixtures and impurities in the solid and liquid phases happens on the phase interface in the solidification interval due to their different solubility. A basic parameter characterizing the distribution of admixtures and impurities between the solid and liquid phases is the segregation coefficient. Its knowledge is important for selecting of a convenient crystallization method of refining, preparation of quality single crystals and study of segregation micro- and macro-inhomogeneities. The growth of single crystals from melts may be accompanied by creating a sequence of structural imperfections in the solid phase which is given by the conditions of crystallization and the crystal chemical composition. A periodical variation of the crystallization rate happens under certain conditions of the heat and mass transfer, which leads to a variation of the character of distribution of admixtures in the volume of the growing single crystal and its substructure. An accumulation of admixtures in front of the crystallization front and increasing of the crystallization undercooling may cause a considerable rise of the speed of movement of the crystallization front and changes of the admixture occupation in the growing crystal on the phase boundary surface. The variation of the microscopic crystallization rate causes a creation of a band structure observed on longitudinal sections of single crystals. The aim of our experimental work was to study an influence of crystallization conditions on the segregation and structural characteristics of single crystals of low-alloyed molybdenum and tungsten alloys. Low-alloyed single crystals of W and Mo doped with Ta were prepared by the electron beam floating zone melting (EBFZM) applying three different zone pass rates - 3, 5 a 1 mm/min. The electron wave dispersive analysis was used for the determination of admixtures concentration on longitudinal sections. On the basis of chemical analysis were found the growth bands in these single crystals that are characterized by the sinusoidal wave of concentration profiles. Further the concentration relations in crystal and freeze zone in order to ascertain of effective segregation coefficients of alloying elements were determined. In order to evaluate relations between chemical inhomogeneity and structural defects and their influence on properties of single crystals there were experimentally investigated.