A photographic technique coupled with image analysis was used to measure the size and fractal dimension of asphaltene aggregates formed in toluene-heptane solvent mixtures. First, asphaltene aggregates were examined in a Couette device and the fractal-like m-re-ate structures were quantified using boundary fractal dimension. The evolution of the floc Structure with time was monitored. The relative rates of shear-induced a,aggregation and fragmentation/restructuring determine the steady-state floc Structure. The average floe Structure became more compact or more organized as the floc size distribution attained steady state. Moreover, the higher the shear rate is, the more compact the floe Structure is at steady state. Second. the fractal dimensions of asphaltene aggregates were also determined in a free-settling test. The experimentally determined terminal settling velocities and characteristic lengths of the aggregates were utilized to estimate the 2D and 3D fractal dimensions. The size-density fractal dimension (D-3) of the asphaltene aggregates was estimated to be in the range from 1.06 to 1.41. This relatively low fractal-dimension suggests that the asplialtene aggregates are highly porous and very tenuous. The aggregates have a Structure with extremely low space-filling capacity. (c) 2004 Elsevier Inc. All rights reserved.