In this work, flying and stationary hot-wire measurements were made to investigate the effect of the Reynolds number on the near- and intermediate-fields region (0 <= x/D <= 25) of a round free jet. Measurements were carried out over a range of Reynolds numbers, based on the jet exit mean velocity and the nozzle diameter, that span the mixing transition. The specific Reynolds numbers tested were 6000, 10,000 and 30,000. The objective of this study was to determine what differences there were in mean velocity profiles, turbulence intensity profiles, and velocity spectra. Results revealed a close coupling between the mean velocity distribution and the turbulence intensities and the Reynolds shear stress. From those data obtained, it was concluded that the inertial sub-range frequency span increases with distance downstream from the jet inlet and the mixing transition seems to occur at the appearance of the inertial sub-range rather than at the transition from the inertial to dissipation range. (C) 2009 Elsevier Ltd. All rights reserved.