In this paper we consider the robustly stabilizing control of a 10001 draft tube baffled crystallizer. When operated at high fines dissolution rates, the crystallizer exhibits sustained oscillations. A detailed population balance model for the process can be found in the literature. Based on this detailed model we develop a simpler population balance model. This, in turn, permits the derivation of an irrational transfer function from manipulated to measured variable. An H-infinity mixed sensitivity minimization problem is formulated and solved using an infinite-dimensional version of H-infinity theory. Two different controllers are designed and compared in simulation studies.