The kinetics of aggregation of soluble constituents of coal, pyridine insoluble (PI), but CS2/NMP mixed solvent soluble fraction were studied by directly observing the formation of insolubles from the solution as a function of time. It was clarified that the aggregation of PI molecules in NMP and in the mixed solvent solution was a reaction limited aggregation (RLA) mechanism, and a two-step kinetic process was provided which was the formation of elemental aggregates and the clustering of these elemental aggregates. The turning point was at about 10 wt.% formation of insolubles. The rate law and rate constants for the aggregation at different holding temperatures and solvents were determined and the rate constants follow the Arrhenius equation. The activation energy of the aggregation of PI molecules in NMP solution was 73.3 and 21.6 kJ/mol for the first step and the second step kinetic process, respectively. The formation of elemental aggregates had a smaller rate constant and higher activation energy, indicating that the first step was the aggregation controlled step. The influence of solvent on the aggregation kinetics was also discussed. (C) 2004 Elsevier B.V. All rights reserved.