The H2O2 concentration for the nonlinear process of the iron-catalysed hydrogen peroxide decomposition in a continuous stirred tank reactor is estimated using a time-variant Extended Luenberger-Observer. The asymptotic stability of the observer, designed by adequate pole-placement, is demonstrated by simulation and comparison of the transient behavior of the estimated concentration profiles with those of the nonlinear system. Comparison with measured H2O2 concentrations indicates satisfactory observer dynamics and accuracy with respect to a lower stationary state of the nonlinear process.