Abetmet-A heterogeneous one dimensional model which takes separately into account the heat transfer through the solid and fluid phases is introduced. The response of this model is compared with that of the heterogeneous two-dimensional model, and a very good agreement, especially at mild conditions, is obtained. Its performance is better than previous one dimensional models and the deviations are explained in terms of the operative conditions and of the value of dimensionless groups. Conditions at which the different one and two-dimensional models are recommended to be used are p-ted. 1. lNTRODKI'IbN The development of mathematical models for the simulation of non adiabatic fixed bed catalytic reactors has received considerable attention. In a previous article (Fereira Duarte et al., 1984) a comparison of the response obtained with different two-dimensional models was carried out. Three. types of models were considered: (I) pseudohomogeneous, (II) heterogeneous, written in the most usual but incorrect way; and (III) heterogeneous, written in the correct way, namely the heat transfer through the solid phase is included in the heat balance of that phase. In that comparison the importance of the two-dimensional type (III) model is emphasized. Although it has seldom been used (DeWasch and Froment, 1971, Holton and Trimm, 1976), it represents correctly, for this kind of reactors, the heat and mass transfer phenomena in and between phases. When the partial differential equations that describe the two-dimensional models are integrated on a cross section of the reactor, ordinary differential equations arise. These describe the one-dimensional models, that can also be classified as mentioned above. We will refer to each of the models with a number (I, II, or III) depending on the type and with a letter (0 or T) to differentiate the one-dimensional from the twodimensional model. Up to now the three two-dimensional models have been used in the literature, but only types I and II of the one-dimensional models. The objective of the *Author to whom correspondence should be addressed. present work is to'introduce model III-O and to analyze the results obtained in comparison with the twodimensional model III-T and with the onedimensional models I-O and II-O.