The previously carried out internal coating of contact tubes not only required very much manual labor and was very time consuming, but it was also unsatisfactory in regard to adhesion of the contact liquid to the surface and homogenity of the catalyst. By the simultaneous action of heat on the liquid interface in the tubes during the lowering of the liquid level as well as by a specially suited apparatus for this purpose which operates according to the principle of communicating tubes there is provided the possibility of a good adhesion to the surface as well as the formation of a homogeneous layer at very little operating expense.