A wetted-wall cylindrical flow reactor was used to measure uptake coefficients, gamma, of O-3 on aqueous surfaces at 293 K. The loss of O-3 from the gas-phase following contact with varying areas of aqueous surface was determined by UV absorption. The use of Na2S2O3 as an aqueous-phase scavenger for O-3 ensured that uptake coefficients were in a reaction-controlled rather than mass accommodation-controlled regime. Observed uptake coefficients were corrected for radial gas-diffusion to yield values of gamma(corr). From extrapolation of a plot of 1/gamma(corr) against the inverse square-root of the Na2S2O3 activity, a value of alpha = 4 x 10(-2) was derived for the true mass accommodation coefficient of O-3. Evaluation of uncertainties indicate a conservative lower limit of 10(-2) for alpha The data do not rule out that the upper limit approaches unity. However, it is shown that the measured value of alpha is sufficiently high that mass accommodation does not limit heterogeneous processing of O-3 in the atmosphere for droplets of diameter >10 mum. A value of 3.7-0.6+0.7 x 10(8) L mol(-1) s(-1) is derived for the aqueous-phase reaction rate coefficient between O-3 and Na2S2O3 at 293 K.