Within industrial applications it is rare to operate wholly under steady-state conditions and there will always be at least one time-dependent parameter. Such transients, in flowrate or pressure, can occur over a short time period, in the order of seconds or minutes, or evolve over a more substantial time period of hours, or even days. The longer this time frame, the less severe impact the transient will have on the operation of equipment. If the flowrate is subjected to a sudden change, any equipment must be either capable of responding quickly to adapt to the variation or be able to absorb the change without adversely affecting the overall system performance. Examples of general transient situations, involve plant shutdown and start-up, changes in flowrates in response to planned operating conditions, like bringing another oil well on line, and emergency situations. Baker et al. [2007. Controlling the phase separation of gas-liquid flows at horizontal T-junctions. American Institute of Chemical Engineers Journal 53, 1908-1915] suggested a novel development of T-junctions for compact phase separation applications based correctly on steady-state observations. However, since it must be expected that transients are an unavoidable problem within industry, this shortfall in knowledge has to be a concern and a probable reason why T-junctions are not used in situations where they could be advantageous. A survey of available literature shows that very little work has been conducted under transient flow conditions. As such, this initial study provides insight into the complex mechanisms and dynamics that occur when transient two-phase flows enter a specific T-junction separator. (c) 2007 Elsevier Ltd. All rights reserved.