In this paper, the extension to tubular reactors of asymptotic observers originally developed for STR is first discussed and illustrated with real-life data on an non-isothermal fixed bed reactor. Then it will be shown how to design exponential observers, which are also independent of the process kinetics but for which the rate of convergence may be arbitrarily fixed (unlike the asymptotic observers for which the rate of convergence depends on the operating conditions). (C) 2000 IFAC.