Use of bubble columns as photobioreactors requires a quantitative knowledge of radial mixing in these columns. A complete model of liquid-phase dispersion was used to simultaneously characterize axial and radial mixing in a relatively large (0.06 m(3), 2.3 m tall, 0.193 m in diameter) bubble column photobioreactor. Axial and radial dispersion coefficients and mixing times were determined in tap water and sea water for superficial aeration velocities of up to 0.051 m s(-1). The measured axial dispersion coefficients (D-z) were generally consistent with the predictions of the well established correlations, thus validating the complete dispersion model used in the analysis. The D-z values ranged from similar to 150 to 380 cm(2) s(-1) and were highly reproducible. There was evidence that the existing literature data on D-z in bubble columns are slightly underestimated, as consistent underestimation was found to be a characteristic of the widely used dispersion model that disregards radial dispersion. The value of the radial dispersion coefficient was typically about 1% of the D-z value under any given condition. Except at incipient aeration, the radial dispersion coefficient was not as sensitive to the magnitude of the aeration rate as was the axial dispersion coefficient. The mixing time data were generally consistent with the existing correlations. (C) 2004 Elsevier Ltd. All rights reserved.