A small break in a horizontal coolant pipe is investigated. This flow geometry and accident scenario are of interest in nuclear reactor safety research. For the calculation of break mass flow rate, appropriate experiments are needed, especially for the case where stratified two-phase flow exists in the main pipe. The flow geometry corresponds to a “T”-junction with a large-diameter ratio of the horizontal pipe, D, to the branch pipe, d. In the present experiments, D was 206 mm, the downward-oriented branch diameters were 6, 12, and 30 mm. Air/water experiments were performed at a system pressure of 0.5 MPa and various differential pressures. The flow field could be observed visually. Photographs reveal both vortex-induced and vortex-free gas pull-through the break and the corresponding correlations for the onset of gas pull-through. The mass flow rate and quality distribution as a function of a dimensionless interface level are presented.