A device for disinfecting of gases and/or liquids, includes a tube of UV-transparent glass having a hollow interior space and a tube wall with a tube inside wall and a tube outside wall, as well as at least one UV-light source. The UV-transparent glass tube has an indentation extending into the interior space on at least one location and in the at least one indentation at least one UV-light source is arranged. The geometry causes the UV-light sources to be closer to the medium to be disinfected, so that a large portion of the UV-light reaches the interior space on a direct path through the glass, thus allowing for a low-loss transfer of the UV-light.