The influence of particle size on the kinetics of slow Brownian coagulation is studied, Colloidal gold was chosen as a model system of spherically shaped colloidal particles with a rather sharp particle size distribution, An experiment shows that dispersions with particle sizes of nanometer scale may deviate from expected behavior according to DLVO theory, This experiment confirms rather well the expectation of a qualitative influence of surface irregularities on log W-log c diagrams. The surface roughness effect has been described in such a way that a transition in the character of the stability diagram would be expected at L = lambda(2)/delta d = 1, This parameter compares the particle diameter with surface roughness characteristics lambda (half-wave) and delta (half-amplitude). This indicates that the surface features commensurate with the range of interparticle forces are more important in determining the rate of slow Brownian coagulation of electrostatically stabilized colloids than the average particle size.