This work deals with the design and the application of a robust control to a chemical reactor. The reactor is exothermic one. There are two parameters with only approximately known values in the reactor. These parameters are the reaction enthalpies. Because of the presence of uncertainty, the robust output feedback is designed. Two robust controllers are designed, the first one is based on the small gain theorem and the second one uses the H2/H∞ control techniques. The presented experimental results confirm applicability of mentioned approaches to safe control of nonlinear processes.