Some mechanisms of flow separation at a vertical tee junction with a horizontal outlet are examined in single-phase and two-phase annular flows. In single-phase flow, some characteristics of the vena contracta at the vertical exit and the shape of the dividing streamline boundaries at the tee entrance are measured. In two-phase flow, experiments carried out with cotton threads indicate, that at low branch take-off a large fraction of the liquid film flow entering the branch is rejected to the tee then entrained to the vertical exit. At high branch take-off, some of the liquid in the vertical exit falls back into the tee junction. This phenomenon is examined with a liquid dye and a high speed video camera, indicating two causes for the liquid to fall along this tube and providing some information on the subsequent division of the falling flow. Finally, the predictions of several flow separation models are compared with the present data and the validity of some assumptions made in mechanistic models is reviewed in relation with the observations provided by the present experiments.