The change of solution viscosity with time for one kind of coal solubles PI-1, which was obtained from the pyridine insoluble (PI) fractions of Upper Freeport (UF) coal extracts with carbon disulfide-N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent (1:1 by volume) by solvent treatment of PI in NMP with additive, tetrabutylammonium acetate (TBAA), was investigated in this study. The fractal dimensions of PI-1 aggregates in solutions were calculated from Pal-Rhodes equation. The results show that the fractal concept can be used to study the aggregation of coal molecules in solution. The aggregation of PI-1 in solution at low temperature is a reaction-limited aggregation (RLA) mechanism, and the fractal dimension of PI-1 aggregates decreases with the increase of temperature, suggesting that the aggregation kinetic process is faster for higher temperature. (C) 2004 Elsevier B.V. All rights reserved.