Abstract Based on the planar assumption, the solute boundary layer in lamellar eutectic growth in steady-state is estimated by means of modifying the conservation of mass at solid/liquid interface. It is found that the solute boundary layer along the growth direction is related to that along the solid/liquid interface. For transparent organic system CBr4-C2Cl6 of eutectic composition, the difference in average composition of the solids and that of the liquid at solid/liquid interface increases with increasing in growth velocities. The average composition in the liquid at solid/liquid interface still closes to the eutectic one as the initial composition of the alloy is apart from the eutectic point. In addition, for a fixed growth rate, the variation of ratio of the difference in average composition to the lamellar widths of the two solid phases with initial composition of the alloy is noticeably distinct.