Countercurrent two-phase flow in horizontal and inclined channels, connecting a sealed liquid-filled reservoir to the atmosphere, is experimentally studied. This type of gravity-driven countercurrent two-phase flow can occur during the operation of passive safety coolant injection systems of advanced reactors. It can also occur in the pressurizer surge line of pressurized water reactors during severe accidents when the hot leg becomes voided. Four distinct flow regimes are identified: (a) stratified countercurrent, which mainly occurs when the channel is horizontal; (b) intermittent stratified-slug; (c) oscillating, which occurs when the angle of inclination is ≥30 deg; and (d) annular countercurrent. The characteristics of each regime and their sensitivity to important geometric parameters are examined. The superficial velocities in the stratified countercurrent and oscillating regimes are empirically correlated.