Apresenta-se um estudo sobre a fragilização por hidrogênio em ferros fundidos nodulares e maleáveis pretos. O trabalho experimental envolveu ferros fundidos nodulares e maleáveis pretos com diferentes microestruturas (ferrita, perlita esferoidizada, ferrita + perlita, perlita com fases intercelulares), introduzindo-se hidrogênio por decapagem e por carregamento catódico. Os resultados experimentais permitiram verificar que a introdução de hidrogênio favoreceu a ocorrência de modos frágeis de fratura, modos estes que a microestrutura tenha tendência a apresentar (fratura por clivagem em ferrita e perlita e fratura intergranular em materiais ferríticos nos quais proporcionou-se a segregação de fósforo para contorno de grão). As trincas de clivagem bem como as áreas de fratura intergranular tendem a situar-se em tôrno dos nódulos de grafita. As evidências experimentais indicam que a localização preferencial de trincas de clivagem e de fratura intergranular junto aos nódulos de grafita seria devido, principalmente, ao acúmulo de hidrogênio provocado pela concentração de deformação plástica nestas regiões. / A study carried out on hydrogen embrittlement in ductile cast irons and blackheart malleable irons is presented. The experiments were made with ductile cast irons and blackheart malleable irons with different microstructures (ferrite, spheroidized pearlite, ferrite + pearlite, pearlite with intercelullar phases), the hydrogen being introduced by pickling and by cathodic charging. The results obtained have shown that hydrogen promotes the occurrence of brittle modes of fracture, whose modes the microstructures had a tendency to present (fracture by cleavage in ferrite and pearlite, and intergranular fracture in ferritic materials in which the segragation of phosphorus to grain boundaries was promoted). The cleavage cracks as well as the intergranular fracture área were more prone to forma round the graphite nodules. The experimental evidences show that the prefered localization of the cleavage cracks and intergranular fracture around the graphite nodules resulted mainly from the accumulation of hydrogen caused by the concentration of plastic deformation in these areas. Thus, in ductile cast Irons and blackheart malleable Irons, due to the stress concentration effect caused by the graphite nodules, under condititons of previous introduction of hydrogen followed by the mechanical stress, the previous distribution of hydrogen at interfaces or grain boundaries would not be the main factor in hydrogen embrittlement (fracture mode and localization of hydrogen induced cracks). The principal effects would be the concentration of stresses and plastic deformation near the graphite nodules, the contribution of the matrix in redistributing local strains and the tendency of the microstructure to the occurrence of brittle fracture mechanisms. Based on this study some suggestions are indicated for the selection of cast Irons for applications where hydrogen is present.