Optimization of cooling systems within gas turbine engines is of greatinterest to engine manufacturers seeking gains in performance, efficiency, andcomponent life. The effectiveness of coolant delivery is governed bycomplex flows within the stator wells and the interaction ofmain annulus and cooling air in the vicinity of therim seals. This paper reports on the development of atest facility which allows the interaction of cooling air andmain gas paths to be measured at conditions representative ofthose found in modern gas turbine engines. The test facilityfeatures a two stage turbine with an overall pressure ratioof approximately 2.6:1. Hot air is supplied to the mainannulus using a Rolls-Royce PLC Dart compressor driven by anaero-derivative engine plant. Cooling air can be delivered to thestator wells at multiple locations and at a range offlow rates which cover bulk ingestion through to bulk egress.The facility has been designed with adaptable geometry to enablerapid changes of cooling air path configuration. The coolant deliverysystem allows swift and accurate changes to the flow settingssuch that thermal transients may be performed. Particular attention hasbeen focused on obtaining high accuracy data, using a radiotelemetry system, as well as thorough through-calibration practices. Temperature measurementscan now be made on both rotating and stationary diskswith a long term uncertainty in the region of 0.3K. A gas concentration measurement system has also been developedto obtain direct measurement of re-ingestion and rim seal exchangeflows. High resolution displacement sensors have been installed in orderto measure hot running geometry. This paper documents the commissioningof a test facility which is unique in terms ofrapid configuration changes, nondimensional engine matching, and the instrumentation densityand resolution. Example data for each of the measurement systemsare presented. This includes the effect of coolant flow rateon the metal temperatures within the upstream cavity of theturbine stator well, the axial displacement of the rotor assemblyduring a commissioning test, and the effect of coolant flowrate on mixing in the downstream cavity of the statorwell.