A one-dimensional model of catalyst-coated fin in plate-fin heat exchanger reactors was developed in this study to consider the effects of heat of reaction on the fin efficiency. The results of the model showed that the occurrence of a surface reaction must be treated carefully since it drastically changes fin behaviour compared to conventional thermal fins. The impact of dimensionless numbers representing the coupling between fin geometry, surface reaction and heat and mass transfer was investigated, and limit conditions where a reactive fin behaves as a conventional thermal fin was circumscribed according to these parameters. The results of the developed reactive fin model were also compared to computational fluid dynamics simulations and showed very good representativeness for all heat transfer, reaction and by-pass effects. The comparison demonstrated that this simplified model is strong enough to represent reactive fins on the surface of which a catalytic surface reaction occurs, for both laminar and turbulent flows in square millichannels.