Title: Hölder-differentiability of Gibbs distribution functions Abstract: In this paper we give non-trivial applications of the thermodynamic formalism to the theory of distribution functions of Gibbs measures (devil's staircases) supported on limit sets of finitely generated conformal iterated function systems in $\mathbb{R} $. For a large class of these Gibbs states we determine the Hausdorff dimension of the set of points at which the distribution function of these measures is not $\alpha$-Hölder-differentiable. (This is joint work with B.O. Stratmann)