The aim of this work is to show how to design a soft-sensor to monitor the Selective Catalytic Reduction (SCR) of NOx with NH3 in a loop reactor, i.e. a loop-shape system made of N units with periodical variation of the feed position. The design procedure of Zeitz [M. Zeitz, Nonlinear observers for chemical reactors, Ph.D. Thesis, VDI-Fortschr.-Ber., Reihe 8, No. 27, VDI-Verlag, Dusseldorf, 1977] is used and, even if the design procedure remains basically heuristic, some general guidelines to design the observer are given. The proposed observer is demonstrated to allow for a quick and reliable estimation of the inlet NOx concentration, as well as of the outlet reactants conversion and of the temperature profiles in the reactors, using some temperature measurements in the reactor, even when the NOx and the NH3 concentration in the feed change. The results may be used for control purposes, thus avoiding expensive hardware sensors and time consuming on-line measurements. (c) 2006 Elsevier B.V. All rights reserved.