Even though Iannis Xenakis never made "musique concrète" in the sense given by Pierre Schaeffer, the GRM was a locus for experimenting with his ideas about sound and sound structures. The works, composed between 1958 and 1962, show a boldness as advanced as in his orchestral approach. The relationship between Xenakis and Schaeffer was often tense. It nevertheless entailed mutual recognition and respect towards each other’s musical approach. Moreover, "Bohor" is dedicated to Schaeffer, as evidenced by the letter Xenakis sent him on October 8th, 1968: "Dear Pierre, I have decided to dedicate Bohor to you, out of friendship and gratitude for your contribution to music. I hope you are pleased with this." Schaeffer found the piece disproportionate in terms of intensity but was indeed pleased by the dedication. The four pieces presented here, all produced at the GRM, undoubtedly demonstrate the experimental intent and the strictly "physical" character of Xenakis' music, in that it provides the audience with a listening experience of a rare intensity. François Bonnet & Christian Zanési