A kinetic analysis in a variety of conditions (gas composition and temperature) has been conducted on catalysts of iron oxides supported on activated carbon fiber composites (ACFC). Additionally, experiments of temperature-programmed desorption (TPD) of NO were conducted on the catalysts in order to reveal mechanistic features of the low-temperature SCR reaction. In the light of current SCR literature and previous work, a qualitative picture of the catalytic behavior of low-temperature SCR catalysts is offered. Apparently, the strength of adsorption of NO during the low-temperature SCR reaction is responsible for the governing reaction mechanism. Thus, highly stable nitrates formed on the surface provoke catalyst deactivation and reaction through an ER mechanism, with NO reacting from the gas phase, whereas the absence of these nitrates permits reaction of less stable nitrites from the catalyst surface, following an LH-type mechanism. This is the case for the ACFC-supported iron oxide catalyst analyzed in this work.