Musical pieces composed for a percussion ensemble form a significant portion of Iannis Xenakis’s creative achievements. One of major compositions in this genre is his Persephassa (1969) for six percussionists. The set of instruments used by Xenakis is divided into four sound levels: skins (peaux), pieces of wood (bois), metals (metal), effects (pierre). Each percussionist has a wide acoustic array consisting of over a dozen instruments. The sonoric aspect of the piece is enriched through the positioning of the musicians around the audience. The first part of the paper analyzes the form of the composition: its course, division, structural, rhythmical, textural, agogic, energy-containing, and sound solutions, as well as the kinds of culminations and special effects (rhythmical chaos). The other part of the article describes the use of compositional devices that determine the specificity of a musical piece, such as complicated rhythmical structuralism, (resulting from the combinations of elements of probability calculus and computer algorithms) or sonoric effects (e.g. “sound clouds”), discussed in close connection with the problems of performing technique.