A method is described for decorating structures in solid aluminum by precipitation of hydrogen onto them. Radioactive hydrogen is employed so that the deposition sites can be revealed by the use of autoradiographic techniques. The study was conducted with zone refined aluminum, and the primary structure that was revealed was that remaining after directional solidification during the last zone pass. The tritium was added to the specimen at 600°C and its precipitation was brought about by quenching the sample. As little as 0.05 ppm of tritium was sufficient to reveal complex structures in the cast metal that are attributed to association of unidentified residual impurity atoms with low angle subgrain boundaries that are thought to have resulted from polygonization occurring immediately behind the advancing solid-liquid interface. Wide-angle grain boundaries were also decorated. New structures were introduced by various levels of strain that accompanied different upsetting operations. Grain and subgrain boundaries that developed during strain anneals were "antisites" for hydrogen deposition. This opposite behavior is attributed to the absence of impurity atoms in these "new" boundaries. An explanation for the behavior of hydrogen in aluminum is given in terms of the condensation of vacancies vs their loss to vacancy sinks. These phenomena are shown to be involved in the enlargement of hydrogen-filled pores in aluminum. © 1970 The Minerals, Metals & Materials Society - ASM International - The Materials Information Society.