A method for carrying out endothermic or exothermic gas phase reactions by using a tube bundle reactor with a tube bundle of catalyst-filled reaction tubes comprises the following steps: a) Introducing a reaction gas mixture into the reaction tubes; b) Dividing-up of the reaction gas mixture flow flowing through each of the reaction tubes into at least two partial flows, each partial flow having the same composition; c) Feeding-in of each partial flow at a different point along the catalyst filling with an existing flow resistance; d) Determining the desired partial flow volume for each partial flow (V.sub.1, V.sub.2, V.sub.3, V.sub.4); e) Calculating the pressure at the point of the first division of the reaction gas mixture (9); f) Calculating the pressure in the catalyst filling (12) at the point of feeding-in of each partial flow (V.sub.1, V.sub.2, V.sub.3, V.sub.4); and g) Setting of flow resistance for each point of feeding-in in such a way that the flow resistance at the desired partial flow volume corresponds to the pressure difference between the pressures determined in steps e) and f).