During the analysis of birefringence inhomogeneities due to axially compositional variations of Li 2O in Czochralski-grown LiNbO 3 crystals, compositional inhomogeneities were revealed which could not be explained by an axial distribution from a well-mixed melt following Pfann's segregation. These inhomogeneities in the first-to-freeze region, crystallized fraction g<0.1, locally resolved observable by orthoscopy, were also found in LiNbO 3 crystals grown from a melt with a well-established congruent melt composition (±0.005 mol% Li 2O). The observed δ n e inhomogeneities showed two typical features: the existence of a Li 2O-rich skin and the formation of a local maximum of extraordinary refractive index n e at 0.045⩽ g⩽0.065. The first is due to Li 2O indiffusion of preferentially evaporated Li 2O from the melt and the second is due to build-up of an axial Li 2O concentration profile in the melt. From calculations based on a simple model, it can be shown that this concentration profile is due to the incongruent evaporation of Li 2O from the melt and the separation and interaction of different convective stream modes in the melt: a surface near convection mode due to the axially thermal gradient near the melt surface and the Marangoni convection — and the thermal convection in the bulk of the melt. The compositional inhomogeneity is axially limited by the change of the convective stream pattern due to dominating forced convection in the melt.