Pousseur studied at the Liège Conservatory from 1947 to 1952 and the Brussels Conservatory from 1952 to 1953. Influenced by such composers as Anton Webern (Webern, Anton), he wrote serial music (serialism), in which various musical elements are rigidly controlled. Yet he also composed aleatory music, involving many types of highly unpredictable events. In Répons pour sept musiciens (1960; “Responses for Seven Musicians”), the course of the composition is partly determined by lottery and by the players' free choice based on moves on a checkerboard. In Pousseur's operalike Le Miroire de votre Faust (1961–68; “The Mirror of Your Faust”), the Faust story is given new twists; which one of four possible denouements a particular performance presents is determined by audience vote.

Pousseur's later musical works include Couleurs croisées (1967; Crossed Colours), a series of unusual variations on the civil rights song "We Shall Overcome" ; its sequel, Croisées des couleurs croisées (1970; Crosses of Crossed Colours), for female voice, pianos, tape recorders, and two radio receivers; Invitation à l'Utopie (1971); Liège à Paris (1977; “Paris Cork”); Le Seconde Apothéose de Rameau (1981; “The Second Deification of Rameau”), for chamber orchestra; and Traverser la forêt (1987; “Crossing the Forest”), a cantata for speaker, two vocal soloists, chorus, and 12 instruments.

Pousseur taught music widely in Europe and the United States. He also helped establish electronic-music studios in Cologne, Ger. (1954), Milan (1956), and Brussels (1958). In his theoretical writings, such as Fragments théoriques I sur la musique expérimentale (1970; “Theoretical Pieces I: Experimental Music”), he argued that older methods of discussing and appraising music are in some instances not valid for music that makes use of new musical aims, resources, and techniques.