Using fluorescence correlation spectroscopy (FCS) we measure the translational diffusion coefficient of lino coal asphaltenes and UG8 petroleum asphaltenes dispersed in toluene at extremely low concentrations (0.03-3.0mg/L). Using a simple scaling argument we compare them with several molecules of known size and molecular weight. We find that lino coal asphaltene molecules-which have a comparatively small alkane fraction-have a diffusion constant of 1.5 x 10(-5) cm(2)/s and corresponding hydrodynamic radius close to 0.25 nm: making them significantly smaller than petroleum asphaltenes. (c) 2006 Elsevier Ltd. All rights reserved.