The main objective of the present work is to study the mass extraction in a transonic wind tunnel. The strategy consists in the analysis of the flow in the test section, with special attention devoted to the slots. To this end the results of tests of a NACA 0012 airfoil are presented. Besides, the influence of the returning flaps and forced evacuation compressors upon the results were also assessed. The experimental investigation used both the classical static-pressure taps as well as the “Pressure Sensitive Paint” (PSP) techniques. The PSP complete pack includes the devices to apply the special ink, dedicated computer codes, and the electronic equipments necessary in the usage of the technique. This technique permits the measurement of the static pressure along all the surface of the body, and further allowed for the observation of shock waves. To complete the study, the flow inside the test section is also numerically simulated. In overall terms the tunnel stream, the pressure distribution on the airfoil and the flow through the slots are investigated. Values of pressure on the surface of the foil were compared including here the present experimental and numerical results, plus data from the literature what guarantees the proper accuracy of all the investigation performed.