This paper deals with the design of an interval observer for plug flow reactors that are modeled by first-order hyperbolic partial differential equations. This model is considered to obtain the dynamics of an auxiliary variable that are further represented by a set of ordinary differential equations by means of the method of characteristics. Regarding these equations, the structure of the interval observer is deduced. This proposed estimation scheme is valid at a certain fixed specific axial point, is robust in the face of both uncertain parameters and load disturbances and is successfully implemented through simulations on an isothermal plug flow reactor within which a consecutive reaction is taking place and on a nonisothermal plug flow reactor with lateral feed where an irreversible reaction is carried out. (c) 2010 Elsevier Ltd. All rights reserved.