Abstract: Reaction segment for a colorimetric analyzer is removably mounted on a revolving plate of the analyzer so that the reaction vessels of the segment are brought one after the other into contact with a measuring device of the analyzer. The segment consists of transparent vertical reaction vessels which are rigidly fixed to a horizontal bar having connected thereto a horizontal plate to which is affixed a device for positioning and guiding into loading and unloading devices of the analyzer and into the opening of the revolving plate.