In crude oil-toluene solutions dynamic viscosity eta and spin-spin relaxation time T-2 were measured as functions of asphaltene concentration C in the range of 10-300 mg/l. The results were supplemented by studies of optical absorption. Extrema in the measured concentration dependencies are attributed to a transient predominance of specific asphaltene aggregates-from dimers to,stacked molecular nanoclusters (MNCs) of four monomers. The strikingly non-ideal properties of solutions with C below 150-170 mg/l are attributed to the strong interactions between asphaltene species. At higher C the solution properties are evidently determined by the weaker interacting MNCs and the concentration effects are closer to those in ideal fluid mixtures. The observed re-entrant n(T2) behaviour in asphaltene solutions hypothetically has the same origin as in supercooled or demixing molecular systems, where anomalous structural and dynamical features are often explained by emerging heterogeneity due to transient spontaneous clustering. (C) 2002 Elsevier Science Ltd. All rights reserved.