Improved reactors for catalytic, exothermic gas-phase reactions having, viewed in the flow direction of a feed gas, an inlet zone (1), a reaction zone (2) containing at least one catalyst (4), and an outlet zone (3) for the product gas, are described. The reactors are provided in the inlet zone (1) or in the inlet zone (1) and the reaction zone (2) with an insulating liner (6) and/or apparatuses for the transport of cooling media and/or the interior walls of the reactor in the inlet zone (1) or in the inlet zone (1) and the reaction zone (2) consist of inert material. The insulating liner and/or cooling media reduce heat transport from the reaction zone (2) into the inlet zone (1) and thus reduce the risk of preignition of the feed gas mixture used or occurrence of undesirable secondary reactions in the inlet zone (1).