Musical form is a central issue in the discussion between Boulez and Stockhausen. This discussion, through a dense correspondence and essays, reflects the changes the notion of musical material underwent in European serial music up to its culmination in the mid 1960s. This mutation is examined in three steps. From the basic formulation of integral serial music in Boulez’s Structures and Stockhausen’s Studie I; through a reconsideration of the hierarchy between the parameters of pitch and rhythm in Boulez’s Le Marteau sans maître and Stockhausen’s Gesang der Jünglinge and Gruppen; to a shift in the notion of material from production to placing as a consequence of reflections on the treatment and perception of sound and their incidences on the shaping of time in Stockhausen’s Mikrophonie I and Boulez’s Éclat.