text: Resumen : Thermo-optical diffusivities, D, were measured for Hamaca, Cerro Negro and Furrial asphaltenes in both toluene and tetrahydrofuran. In toluene a plot of D vs. log c where c was the asphaltene concentration, afforded an unexpected minimum near 50 mg l21. Similar plots in THF for Hamaca asphaltenes yielded the result with little or no change in D with concentration changes. The reduction in D before the minimum (c , 50 mg l21in toluene) is consistent with the trapping of a layer of solvent between solute molecules (sort of solute–solvent aggregates) presumably resulting in a higher heat capacity for the solvent in this aggregate when it is compared with the solvent in the bulk. At higher concentrations, and probably because of the collapse of the solute–solvent aggregates and the formation of solute–solute aggregates (dimers, trimers, etc.), solvent is released to the bulk, leading to increases in D until a fairly constant value is reached near 2000 mg l21. 1999 Elsevier Science Ltd. All rights reserved.