The invention relates to a reactor for testing a catalyst system, comprising a plurality of parallel contact tubes (2) which are arranged in the inner chamber of the reactor, the ends of said contact tubes being welded to tube bottoms. Said reactor also comprises hoods (3) at the two ends thereof, said hoods respectively defining a hood space (4). A liquid reaction medium (5) is supplied to the contact tubes (2) via a hood space (4), flowing through the contact tubes (2) and being transported away via the other hood space (4). The reactor also comprises a heat exchanging agent circuit with the heat exchanging agent (6) supply at one end of the reactor, the throughflow of the intermediate area between the contact tubes (2), and the discharge at the other of the reactor, the contact tubes (2) being arranged in at least two contact tube regions which are thermally separated from each other.