Slabs produced by hot rolling may receive a high temperature soaking treatment before they are finally formed. In this work, 28-mm thick slabs, produced in the VAW Aluminum Company, Bonn, Germany, were homogenized at different homogenization temperatures in the range of 550-620°C for 1 hr. The heating process was carried out with different rates whereas cooling was achieved either with the same rates or faster by quenching into water to room temperature. Localized cracks in the one-quarter thickness positions of these slabs were clearly observed after receiving the 600°C treatment when heating and cooling took place with rates20°C/min only. The severity of these cracks increases with increasing cooling rate and they spread over the whole slab thickness when the homogenization temperature is increased. Crack initiation and formation were observed using the optical (OM) and the scanning electron microscope (SEM). Concurrently, differential thermal analysis and thermo-mechanical analysis were carried out to measure respectively the phase transformations and thermal expansion taking place during the homogenization treatment.