A method of varying the temperature of a tube bundle reactor for catalytic gas phase reactions upon start-up and shut-down, the tube bundle reactor comprising a major reactor portion which includes a bundle of vertically disposed reactor tubes, upper and lower tubesheets tightly connected to the upper and lower ends, respectively, of the reactor tubes, and a reactor shell enclosing the tube bundle, a heat transfer medium having a melting temperature in the range of from 100° C. to 450° C. flowing around the outer surfaces of the reactor tubes during normal operation and being circulated in at least one circuit through the major reactor portion, comprising the steps: (a) varying the heat transfer medium temperature during circulation of the heat transfer medium; and (b) passing a temperature gas through the reactor tubes at least when the heat transfer medium is not yet or no longer circulated.