This work is an experimental investigation of the turbulent concentration field formed when the nozzle gas from a round momentum-driven. Free turbulent jet mixes with gas entrained from a quiescent reservoir. The measurements, which were made with a non-intrusive laser-Rayleigh scattering diagnostic at Reynolds numbers of 5000, 16000, and 40000, cover the axial range from 20 to 90 jet exit diameters and resolve the full range of temporal and spatial concentration scales. Reynolds--number-independent and Reynolds-number-dependent similarities are investigated. The mean and r.m.s. values of the concentration are found to be consistent with jet similarity laws. Concentration fluctuation power spectra are found to be self-similar along rays emanating from the virtual origin of the jet The probability density function for the concentration is also found to be self similar along rays Near the centreline of the jet. the scaled probability density function of jet fluid concentration is found to be nearly independent of the Reynolds number.