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The Molecular Tagging Velocimetry (MTV) technique has been widely used for analyzing velocity fields in liquid mini- and microflows. Concerning gaseous flows, only few works describe the implementation of MTV at millimetric scale, and these studies are limited to the analysis of external flows, such as jet flows. The goal of the present work is to develop this technique for the analysis of internal gas flows in minichannels. It is a first step toward the visualization of velocity profiles in rarefied conditions, and direct measurement of velocity slip at the walls. A specific experimental setup has been designed. Its features are detailed. Velocity profiles are obtained in a pressure driven steady flow of argon through a long rectangular minichannel of 1.2 × 5 mm2 cross-section and 15 cm length using acetone molecules as tracer. Experiments are carried out at atmospheric pressure, in a laminar continuum flow regime. The accuracy of the method is discussed by comparison between experimental and theoretical velocity profiles. The potential of the MTV technique for analyzing mini or micro gaseous internal flows is commented on. Perspectives of the work for discussing the validity of boundary conditions in the slip flow regime are presented.