Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Al-Pb alloys with monotectic and hypermonotectic compositions were directionally solidified under unsteady-state heat flow conditions. The cooling curves recorded during solidification allowed solidification thermal parameters such as the cooling rate (T), growth rate (v) and thermal gradient (G) to be experimentally determined. Different microstructural patterns have been associated with the alloy solute content, i.e., Al-1.2 and 2.1 wt% Pb. A sequence of morphologies from the bottom to the top of the Al-1.2 wt% Pb alloy casting (monotectic) can be observed: Pb-rich droplets in the aluminum-rich matrix, followed by a region of microstructural transition formed by droplets and fibers and finally by a mixture of fibers and strings of pearls. A completely fibrous structure (without transition) has been observed along the entire Al-2.1 wt% Pb alloy casting (hypermonotectic). The interphase spacing (l) was measured along the casting length, and experimental correlations between lambda and experimental solidification thermal parameters have been established. Power laws with a -2.2 exponent expressing lambda as a function of the growth rate, v, were found to better represent the fibrous growth of both Al-Pb alloys. Moreover, a single experimental law expressing lambda as a function of both G and v was found to describe the fibrous growth of both the monotectic and the hypermonotectic alloys experimentally examined.304401407Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)