The scalar mixing field of a free, turbulent concentric round jet has been examined using marker nephelometry. The flow conditions included velocity ratios between the centre and annular jet of 0.188, 0.519 and 0.911. The correlation function between the concentration fluctuations in the two jet streams increased from -1.0 near the nozzle to +1.0 further downstream indicating a tendency to complete mixing between the jets. The initial mixing behaviour between the jets was better for the lower velocity ratio between the jets (U-i/U-0 = 0.188) although further downstream there was better transverse mixing between the jets with the higher velocity ratio (U-i/U-0 = 0.911).