While one usually looks for reversible reactions, in reality, many of the reactions are impeded. This turns out to yield interesting results. In particular, certain reaction routes that would seem impossible become possible, as certain a priory competing and thermodynamically more favorable routes are blocked. We review here few cases that were recently discussed in the field of solid state electrochemistry. Yet, the first example is a chemical reaction (fort-nation of Fe3C) recently considered, which serves as an introduction. The other impeded reactions discussed are those that show up in a single chamber or mixed gas solid oxide fuel cell, in the electrocatalytic enhancement of reactions, NEMCA, when the reaction cell is based on an oxygen ion conductor (YSZ) and finally, the catalytic activity (enhancement and retardation of reactions) of certain metallic electrodes in cells based on the proton conductor Nafion. (c) 2005 Elsevier B.V. All rights reserved.