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A parameteric study on the rate expression recently developed by Xu and Froment (1989a, A.I.Ch.E.J.35, 88-96) is carried out over a wide range of parameters. The analysis of this rate expression has shown a non-monotonic dependence of the reaction rate upon steam partial pressure which explains the contradictions between the rate expressions a available in the literature, that is the prediction of positive, as well as negative effective reaction order with respect to steam. A simplified pseudo-homogeneous model with a constant effectiveness factor, with Xu and Froment's rate expression was used to investigate the implications of the non-monotonic kinetics on the performance of steam reformers. The main implication was also checked using a heterogeneous model that represents very closely industrial steam reformers.