•Vapor–liquid refrigerant separation in vertical impact T-junction is studied experimentally.•A simple correlation of liquid falling in upward vertical tube is derived.•Modified models for annular and stratified-wavy flow are developed based on the liquid falling correlation. In this paper, the performance of a vertical impacting T-junction acting as a phase separator for application in thermodynamic cycles was investigated experimentally. The T-junction had a horizontal inlet tube (internal diameter 8.0mm) and two equal-sized vertical outlet branches (internal diameter 17.5mm). All experiments were conducted using refrigerant R134a, with inlet mass flux and quality being varied in the range of 100 to 600kg·m−2·s−1 and 0.1 to 0.6, respectively. Flow regimes prior to the T-junction were identified and characterized. Effects of vapor and liquid velocity as well as the opening of control valve on the liquid separation efficiency were investigated. It was found that the separation efficiency deteriorated dramatically as the vapor phase Froude number in the upward tube increased. Liquid falling phenomenon in the upward branch was observed, and a simple correlation between the fraction of falling liquid and the vapor phase Froude number was derived. On the basis of this correlation, modified models for phase separation of annular and stratified-wavy flow in a vertical impacting T-junction were developed. The average absolute deviation between the predicted liquid mass flow ratio at the bottom outlet of the T-junction and those measured values is about 6.4% and 6.9% for annular flow and stratified-wavy flow respectively.