Binary mutual diffusion coefficients measured by the Taylor dispersion method are reported for aqueous solutions of alpha-cyclodextrin (alpha-CD), 2-hydroxypropyl-alpha-cyclodextrin (HP-alpha-CD), and 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD) at concentrations from (0.002 to 0.010) mol center dot dm(-3) at temperatures from (298.15 to 312.15) K. The hydrodynamic radius and activation energy for the diffusion of aqueous alpha-CD, HP-alpha-CD, and HP-beta-CD are calculated from the experimental results. In addition, the Hartley equation and the measured diffusion coefficients are used to estimate activity coefficients for aqueous alpha-CD, HP-alpha-CD, and HP-beta-CD.