Mechanically and capillary pumped two-phase heat transport systems are currently developed to meet the high power and long transport distance requirements of thermal management systems for future spacecraft. Compared to existing single-phase systems, two-phase loops offer important advantages: reduced overall mass and pumping power consumption, virtually isothermal behaviour, adjustable working temperature, insensitivity to variations in heat load and sink temperature, and high flexibility with respect to the location of heat sources within the loop.As two-phase flow and heat transfer in low-gravity environment is expected to (considerably) differ from terrestrial behaviour, the technology of two-phase heat transport systems and their components is to be demonstrated in orbit. Therefore a Dutch-Belgian Two-Phase experiment has been developed within the ESA In-Orbit Technology Demonstration Programme. TPX is a two-phase ammonia system in the (5ft3, gaseous nitrogen filled) Get Away Special canister G557, to be flown aboard the Space Shuttle.The experiment, being a downscaled capillary pumped two-phase system, includes downscaled versions of mechanically pumped two-phase loop components: multichannel condensers, vapour quality sensors and a controllable 3-way valve for control exercises. The experiment will be running autonomously, using its own power supply, data handling and experiment control, after switch-on by the Shuttle crew.The Payload Acceptance Review status of TPX is discussed: design, manufacture, and test results.