This article aims to document how spectral models became an essential component of Gérard Grisey’s creative approach at the beginning of the 1970s. In Dérives (1974–75), though the spectral model remains rather broad, it determines both the work’s temporal structure and the choice of harmonic polarities and progressions. In Périodes (1974), which marks the beginning of the cycle Les espaces acoustiques (1974–85), Grisey specifically transcribes the harmonic spectrum of fundamental frequency E1 (41.2 Hz) and uses it to govern most compositional processes, thus laying the foundations of the spectral music. This study highlights the importance of data drawn from the spectral (de)composition of sound in the compositional processes of both works. It also reveals how Grisey establishes a number of complex compositional techniques then used to structure each detail in the piece, and how he consequently defines a musical project quite comparable to that of serial music.