A mixing-in device for a reactor for carrying out a reaction between two fluid starting materials (2, 3) over a catalyst bed with premixing of the fluid starting materials (2, 3) before introduction into the catalyst bed within a delay time of less than 150 ms in a mixing-in device, wherein the mixing-in device is made up of the following elements which are arranged essentially transverse to the inflow direction of the first fluid starting material stream: - two or three rows arranged behind one another of tubes which have turbulence generators on the outside thereof, which are finned tubes having an external diameter in the range from 25 to 50 mm, with the second row of finned tubes being arranged opposite the first on gaps and, in the case of three rows of finned tubes, the third row of finned tubes being arranged on gaps to the second row of finned tubes, and which constrict the free flow cross section for the first fluid starting material stream to from 1/2 to 1/10, with the second fluid starting material stream being passed through the interiors of the tubes and injected via openings in the tubes into the first fluid starting material stream, with the turbulence generators being configured as fins and the openings opening into the tubes in the channels between the fins and the finned tubes each having two diametrically opposite openings per channel on the exterior circumference of the tubes forming them in the channels between the fins at the places having the smallest distance to the in each case adjacent finned tube in the row of finned tubes, and - a perforated plate upstream of the tubes and - a perforated plate which is arranged downstream of the tubes and has openings whose diameter is greater than or equal to the diameter of the openings of the upstream perforated plate and the mixing-in device has a depth, i.e. a distance between the preceding perforated plate and the subsequent perforated plate, in the range from 100 to 200 mm. - two or three rows arranged behind one another of tubes which have turbulence generators on the outside thereof, which are finned tubes having an external diameter in the range from 25 to 50 mm, with the second row of finned tubes being arranged opposite the first on gaps and, in the case of three rows of finned tubes, the third row of finned tubes being arranged on gaps to the second row of finned tubes, and which constrict the free flow cross section for the first fluid starting material stream to from 1/2 to 1/10, with the second fluid starting material stream being passed through the interiors of the tubes and injected via openings in the tubes into the first fluid starting material stream, with the turbulence generators being configured as fins and the openings opening into the tubes in the channels between the fins and the finned tubes each having two diametrically opposite openings per channel on the exterior circumference of the tubes forming them in the channels between the fins at the places having the smallest distance to the in each case adjacent finned tube in the row of finned tubes, and - a perforated plate upstream of the tubes and - a perforated plate which is arranged downstream of the tubes and has openings whose diameter is greater than or equal to the diameter of the openings of the upstream perforated plate and the mixing-in device has a depth, i.e. a distance between the preceding perforated plate and the subsequent perforated plate, in the range from 100 to 200 mm.