Al-Pb alloys were solidified under non steady heat flow conditions using a casting assembly in order to promote vertical downward directional solidification. The downward configuration enables the effects of gravity-driven convection on the final microstructure to be evaluated since the collective movement of Pb-rich particles downwards is favored, due to density differences between the two coexisting liquid phases. Investigations have been made of the obtained solidification structures. Growth rates (v) and cooling rates ((T) over dot) of the Al-Pb alloys solidified downwards were experimentally determined by the cooling curves recorded along the casting length. The monotectic structure was characterized by metallography and a microstructural transition has been observed in all cases. The microstructure was characterized by well-distributed Pb-rich droplets in the aluminum-rich matrix from the casting cooled surface up to a certain position in the casting, followed by a mixture of fibers and strings of pearls from this point to the bottom of the casting. The increase in alloy lead content delays the formation of fibers for alloys solidified downwards, which occurs for v < 0.48 mm/s and v < 0.15 mm/s for Al-0.9 wt%Pb and Al-1.2 wt%Pb alloys, respectively. Experimental power laws relating the interphase spacing, lambda, to v and characterized by -2.0 and -6.5 exponents, were found to represent the growth of droplets and fibers, respectively, for both alloys solidified downwards.