The dendritic spacings are important microstructural parameters involved in solidification processes. They can affect not only microsegregation profiles but also the formation of secondary phases within interdendritic regions, which influences mechanical properties of cast structures. A small number of studies have been carried out in order to analyze the effects of melt convection within the interdendritic region or to verify the influence of growth direction in the dendritic arm spacings. In this work, 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 during downward unsteady-state solidification of hypoeutectic Al-Cu alloys. These solidification thermal variables are correlated with dendritic spacings, which have been measured along cross and longitudinal sections of ingots solidified under downward unsteady-state heat-flow conditions. Predictive theoretical models for dendritic spacings have been compared with experimental observations. A comparison between upward and downward unsteady-state results for dendritic spacings has also been conducted. (C) 2004 Elsevier B.V. All rights reserved.3832271282