The objective of this research was to document the relative diffusion widths of ellipsoidal air bubbles in water at very low void fraction for a turbulent mixing layer. This was accomplished by recording the transverse concentration distribution of bubbles in an upward-flowing two-stream, turbulent, planar free shear layer of tap water. Ellipsoidal air bubbles with nominal equivalent diameters of 2.5, 3.5 and 4.5 mm were injected directly into the shear layer and illuminated by a scanning laser beam 12-36 cm downstream. For each test condition, three image intensity distributions (based on long-time exposures which resolved individual bubbles in the shear layer) were recorded. From the digitized images, the average Gaussian dispersion widths of the bubble concentration distributions were measured for three bubble sizes, two sheer layer speeds, and six streamwise locations. The results demonstrate increased bubble concentration width at larger bubble sizes and at higher flow speeds. The former is primarily attributed to the interaction between turbulent diffusion and the deformation-induced trajectory oscillations (inherent to rising ellipsoidal bubbles) while the latter is attributed to the increased ratio of fluid turbulent velocity fluctuations to bubble terminal velocity. (C) 2000 Elsevier Science Ltd. All rights reserved.