Working on Analogique B in the late 1950s, Xenakis formulated the ‘hypothesis of a second order sonority’. This paper focuses on this concept, and shows its crucial meaning in the composition of a number of pieces, such as Concret PH, Analogique B and the very recent Gendy3 and S709. The perspective taken is twofold: on one side, the problem of 2nd-order sonorities is seen as a problem in modelling perceptual attributes (or timbre) of sound in terms of ‘emergent properties of organised sound structure’. This is also related to the composer’s pioneering work with granular sound representations and nonstandard methods of sound synthesis. On the other, the issue is discussed in conjunction with Xenakis’ attempt at implementing thoroughly formalised ‘mechanisms’ or ‘systems’ based on the mathematics of stochastic processes. The paper shows that the issue touched on here is essential to the aesthetics of Xenakis’ electroacoustic music. It also questions whether the formalisation of stochastics is a suitable means to let 2nd-order sonorities emerge from the internal life of the sound matter. The limitations of Xenakis’ ‘mechanisms’ are described in terms of their cybernetics, and are situated in the context of artistic and scientific paradigm shifts concerning the relationship between order, disorder and chaos.