Despite the wide use of Al-Sn alloys for engineering applications studies on the microstructural development of such materials are rare. Optimized microstructures during the solidification stage of processing can be fundamental for final properties. In the present study, three Al-Sn hypoeutectic alloys were directionally vertically solidified under upward unsteady state heat flow conditions. Primary (XI) and secondary (; 2) dendrite arm spacings were measured along the alloys castings and correlated with transient solidification thermal variables. A combined theoretical and experimental approach has been used to quantitatively determine such thermal variables, i.e., transient metal/mold heat transfer coefficients, tip growth rates, thermal gradients, tip cooling rates and local solidification time. The article also focuses on the dependence of dendrite arm spacings on the alloy solute content. Furthermore, the experimental data concerning the solidification of Al 20, 30 and 40 wt% Sn alloys are compared with the main predictive dendritic models from the literature. (c) 2007 Elsevier B.V. All rights reserved.