An apparatus suitable for investigating solid catalysts and processes in which discontinuous fluid streams arise, the apparatus including: a reactant fluid supply point; a reaction space; at least one fluid mixing space; at least one throttle element; at least one pressure control valve; and at least one analyzer. An outlet side of the reaction space is operatively connected to the fluid mixing space via a connecting line and a substream line. The fluid mixing space is connected to the throttle element. The throttle element is operatively connected to the analyzer and an outlet line. The connecting line is operatively connected to the pressure control valve and an exit air line. The pressure control valve is arranged either downstream or upstream of the substream line. When the pressure control valve is upstream of the substream line, the outlet line is provided with a second pressure control valve and a pump.