The kinetics of the dissolution of colloidal gold in aerated cyanide solutions has been investigated with a view to developing a fundamental understanding of the mechanism involved. Experimental results obtained are consistent with an electrochemical mixed potential model, where the mixed potential is set up in the passive or passive interpeak region of the anodic polarization curve, giving rise to a rate of dissolution which is controlled by the rate of a surface reaction. Electron microscope studies suggest that the colloidal particles dissolve in a uniform shrinking particle manner. Mass-transfer coefficients for the transfer of dissolved oxygen to the colloidal surface appear to be very high, with estimates for gelatin stabilized colloids being of the order of 10(-4) m/s, and for unstabilized colloids 0.1 m/s. These latter estimates are in agreement with values predicted from a hydrodynamic treatment of the diffusion layer.