Experiments were conducted to analyze the upward unsteady-state directional solidification of Al-Si hypoeutectic alloys. A combined theoretical and experimental approach is developed to quantitatively determine solidification thermal variables such as transient metal/mold heat transfer coefficients, tip growth rates, thermal gradients, tip cooling rates and local solidification time. The observation of the macrostructures has shown that the columnar to equiaxed transition (CET) occurred essentially at the same position from the casting surface for any alloy experimentally examined. The solidification thermal variables were very similar at the CET and it was not possible to identify a structural transition criterion based on a particular variable. The article also focuses on the dependence of dendrite arm spacings on the solidification thermal variables and alloy solute content. The experimental data concerning the solidification of Al with 3, 5, 7 and 9 wt.% Si alloys are compared with the main predictive dendritic models from the literature. (C) 2004 Elsevier B.V. All rights reserved.38141671168181