In this paper we discuss the choice of sensor positions for a tubular reactor with a preset number of sensors. Different observability measures, based on the observability matrix (Kailath, Linear Systems, Prentice Hall, Englewood Cliffs, NJ, 1980; Callier and Desoer, Linear System Theory, Springer-Verlag, Berlin, 1991; Damak et al., DYCORD ＇92, 1992, pp. 315-320), the observability gramian (Callier and Desoer, 1991) as well as on the Popov-Belevitch-Hautus rank test (Kailath, 1980) will be considered for locating optimal sensor positions. The analysis is carried out on the reduced finite-dimensional model of the process. The results of these investigations will be illustrated in simulation and put in perspective with the modal observability properties of the original infinite-dimensional model.