A reformer which enables rapid load changes of up to 100% within a few seconds and is intended to produce hydrogen from hydrocarbons by steam reformation, comprises an evaporator cooler for cooling the reformate and for generating steam. The evaporator cooler is disposed in the reformer, on the end of its reaction vessel. It keeps the applicable end of the tube cool and uses the waste heat of the reformate for generating steam. This makes fast load changes possible, because an increase in the introduction of water immediately causes an increase in the reformate produced and thus an increase in the heat output.