When a cylindrical container, partially filled with a binary granular mixture of particles that differ in size or density, is rotated around its axis, a spontaneous segregation of the two granular components may occur. In order to better understand this phenomenon, we have carried out an experimental study probing the effect of average particle size and relative size difference between particles on the onset of segregation. The experimental study is followed by a novel scaling analysis which relates the deterministic, convective driving force for particle segregation to the randomizing diffusional driving force present in these systems through the definition of an axial granular Peclet number. Values of this granular Peclet number are shown to successfully correlate with segregation behavior in the present study, as well as in comparable results in the literature. (c) 2006 Published by Elsevier Ltd.