In the first part of this paper, we introduce the notion of δ-dual of a quasiunipotent a b -module and prove a formula for its spectrum. In the second part, we show that the a b -module associated with an isolated hypersurface singularity of dimension n is n + 1 -selfdual. From these two independant facts, we conclude with the classical symmetry property of the spectrum of the singularity.